usb: gadget: FunctionFS: dereference ffs_dev conditionally

ffs_dev->ffs_release_dev_callback should be accessed only if ffs_dev
is not NULL.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>