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
[ac8dde1: “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: <[email protected]>  # 3.14
Reported-by: Lad, Prabhakar <[email protected]>
Reported-by: Krzysztof Opasiak <[email protected]>
Signed-off-by: Michal Nazarewicz <[email protected]>
Signed-off-by: Felipe Balbi <[email protected]>

Author: mina86

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: