usb: gadget: f_fs: fix error handling

This patch add missing error check in ffs_func_bind() function, after
ffs_do_descs() function call for high speed descriptors. Without this
check it's possible that the module will try dereference incorrect
pointer.

[ balbi@ti.com : removed trailing empty line ]

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>