usb: gadget: f_fs: Unlock mutex before unregistering gadget

Release the lock before unregistering gadget item.
Unregistered gadget item will result in unbind call back
to the driver which may use the same lock and resulting
in deadlock condition.

Change-Id: I2f8fba73a831d3bb711fe9310c5758221359819e
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>