iwlwifi: fix remove key error

Fix following error by sending synchronous command and waiting for the command
to complete.

mac80211-phy0: failed to remove key (0, ff:ff:ff:ff:ff:ff) from hardware (-16).

-16 is EBUSY error. The asynchronous command tests for STATUS_EXIT_PENDING
while interface is getting down and it returns -EBUSY error if set.
Changing the host command from asynchronous call to synchronous call
enables command to be run while interface is going down.

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