iwlwifi: make EXIT_PENDING depend on mac80211

There's no reason to set EXIT_PENDING when we
start removing the module, as mac80211 will
cleanly shut down the device in this case.
Additionally, there's no point in rejecting
commands to the device when we're cleaning up
as that only leads to unwanted errors from
mac80211 being printed, such as
  failed to remove key (...) from hardware (-16)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>