usb: gadget: f_fs: resurect usb_functionfs_descs_head structure

Even though usb_functionfs_descs_head structure is now deprecated,
it has been used by some user space tools.  Its removel in commit
[ac8dde11: “Add flags to descriptors block”] was an oversight
leading to build breakage for such tools.

Bring it back so that old user space tools can still be build
without problems on newer kernel versions.

Cc: <stable@vger.kernel.org>  # 3.14
Reported-by: NLad, Prabhakar <prabhakar.csengg@gmail.com>
Reported-by: NKrzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: NMichal Nazarewicz <mina86@mina86.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

include/uapi/linux/usb/functionfs.h

@@ -33,6 +33,13 @@ struct usb_endpoint_descriptor_no_audio {
 	__u8  bInterval;
 } __attribute__((packed));
 
+/* Legacy format, deprecated as of 3.14. */
+struct usb_functionfs_descs_head {
+	__le32 magic;
+	__le32 length;
+	__le32 fs_count;
+	__le32 hs_count;
+} __attribute__((packed, deprecated));
 
 /*
  * Descriptors format: