net: macb: Correct the MID field length value

The latest spec "I-IPA01-0266-USR Rev 10" limit the MID field length to 12 bit
value. For previous versions it is 16 bit value.

This change will not break the backward compatibility as the latest ID value is
7 and with in the 12 bit value limit.
Signed-off-by: NPunnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb.h

@@ -351,7 +351,7 @@
 
 /* Bitfields in MID */
 #define MACB_IDNUM_OFFSET			16
-#define MACB_IDNUM_SIZE				16
+#define MACB_IDNUM_SIZE				12
 #define MACB_REV_OFFSET				0
 #define MACB_REV_SIZE				16