iwlwifi: pcie: clean RFKILL interrupt in AMPG

Newer firmware don't clean the RFKILL interrupt in APMG, do
it in driver instead.
If we forget to do so, we can't send HCMD to firmware while
the NIC is in RFKILL state.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>