iwlwifi: create iwl_mac80211 unregister routine

The mac80211 setup_register operations are collected in one routine,
but the cleanup routines are not.  Create a routine for this.

Signed-off-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>