usb: gadget: f_fs: resurect usb_functionfs_descs_head structure

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.

Reported-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Reported-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Cc: <stable@vger.kernel.org>  # 3.14
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Commit taken from linux-usb mailing list. Not merged yet but
required to fix build of FFS apps.

Change-Id: I90fc3786f043aca9463e255ad9900c2d341fd17b
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>