usb: f_fs: replace BUG in dead-code with less serious WARN_ON
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 <94dddeeef08b001e003cce128ddc162a4e2c6cd2@ti.com>
Signed-off-by: Michal Nazarewicz <605208d9b4c769ca62e9f9f8b592c9cd465fe6d7@mina86.com>
Signed-off-by: Felipe Balbi <94dddeeef08b001e003cce128ddc162a4e2c6cd2@ti.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs