rfkill: Fix incorrect check to avoid NULL pointer dereference

In rfkill_register, the struct rfkill pointer is first derefernced
and then checked for NULL. This patch removes the BUG_ON and returns
an error to the caller in case rfkill is NULL.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Link: https://lore.kernel.org/r/20191215153409.21696-1-pakki001@umn.edu
Signed-off-by: Johannes Berg <johannes.berg@intel.com>