usb: gadget: FunctionFS: Fix missing braces in parse_opts

Add missing braces around an if block in ffs_fs_parse_opts. This broke
parsing the uid/gid mount options and causes mount to fail when using
uid/gid. This has been introduced by commit b9b73f7c (userns: Convert usb
functionfs to use kuid/kgid where appropriate) in 3.7.

Cc: <stable@vger.kernel.org>
Signed-off-by: Benoit Goby <benoit@android.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>