usb: gadget: FunctionFS: Return -ENOENT instead of -ENODEV when device not found.

Syscall mount returns -ENODEV error if requested FS type
has not been found. Returning the same error from FFS mount
callback makes value returned to userspace misleading.

Other file systems returns -ENOENT if requested device
has not been found. Adjust FFS to this convention to make
error codes meaningfull.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>