p54: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200225011846.GA2773@embeddedor

drivers/net/wireless/intersil/p54/eeprom.h

@@ -24,7 +24,7 @@
 struct pda_entry {
 	__le16 len;	/* includes both code and data */
 	__le16 code;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct eeprom_pda_wrap {
@@ -32,7 +32,7 @@ struct eeprom_pda_wrap {
 	__le16 pad;
 	__le16 len;
 	__le32 arm_opcode;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct p54_iq_autocal_entry {
@@ -87,7 +87,7 @@ struct pda_pa_curve_data {
 	u8 channels;
 	u8 points_per_channel;
 	u8 padding;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct pda_rssi_cal_ext_entry {
@@ -119,7 +119,7 @@ struct pda_custom_wrapper {
 	__le16 entry_size;
 	__le16 offset;
 	__le16 len;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 /*

drivers/net/wireless/intersil/p54/lmac.h

@@ -81,7 +81,7 @@ struct p54_hdr {
 	__le16 type;	/* enum p54_control_frame_types */
 	u8 rts_tries;
 	u8 tries;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 #define GET_REQ_ID(skb)							\
@@ -176,7 +176,7 @@ struct p54_rx_data {
 	u8 rssi_raw;
 	__le32 tsf32;
 	__le32 unalloc0;
-	u8 align[0];
+	u8 align[];
 } __packed;
 
 enum p54_trap_type {
@@ -267,7 +267,7 @@ struct p54_tx_data {
 		} __packed normal;
 	} __packed;
 	u8 unalloc2[2];
-	u8 align[0];
+	u8 align[];
 } __packed;
 
 /* unit is ms */

drivers/net/wireless/intersil/p54/p54.h

@@ -126,7 +126,7 @@ struct p54_cal_database {
 	size_t entry_size;
 	size_t offset;
 	size_t len;
-	u8 data[0];
+	u8 data[];
 };
 
 #define EEPROM_READBACK_LEN 0x3fc