iwlwifi: fix unload warning and error

This patch fix the error we get when unload the driver, No space for Tx.
The cause of this problem is related to receiving late SW rfkill from
rfkill subsystem during the driver teardown causing this error.

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