ieee802154: fakelb: switch from BUG_ON() to WARN_ON() on problem

[ Upstream commit 8f2fbc6c ]

The check is valid but it does not warrant to crash the kernel. A
WARN_ON() is good enough here.
Found by checkpatch.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>