usb: f_fs: replace BUG in dead-code with less serious WARN_ON

Even though the BUG() in __ffs_event_add is a dead-code, it is still
better to warn rather then crash the system if that code ever gets
executed.

Reported-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>