iwlwifi: return 0 for AMPDU_TX/RX_STOP request if NIC is going down

When receive IEEE80211_AMPDU_RX_STOP or IEEE80211_AMPDU_TX_STOP request
in iwl_mac_ampdu_action() from mac80211; check STATUS_EXIT_PENDING bit,
if NIC is on the way out, then return 0 back to mac80211, this can
prevent mac80211 report HW error incorrectly.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>