iwlwifi: mvm: replace BUG_ON by WARN_ON in scan.c

While the scan_cmd should really be allocated in init (and
we do fail init in case the allocation failed), it doesn't
mean we should lock up the machine if something really bad
happened.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>