mac80211: do not print WARN if config interface

It is expected that config interface will always succeed as mac80211
will only request what driver supports. The exception here is when a
device has rfkill enabled. At this time the rfkill state is unknown to
mac80211 and config interface can fail. When this happens we deal with
this error instead of printing a WARN.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>