iwlwifi: mvm: fix D3 CCMP TX PN assignment

When going into/coming out of D3, the TX PN must be programmed into
and restored from the firmware respectively. The restore was broken
due to my previous commit to move PN assignment into the driver.
Sending the PN to the firmware still worked since we now use the
counter that's shared with mac80211, but accessing it through the
mac80211 API makes no sense now.

Fix this by reading/writing the counter directly. This actually
simplifies the code since we don't need to round-trip through the
key_seq structure.

Fixes: ca8c0f4b ("iwlwifi: mvm: move TX PN assignment for CCMP to the driver")
Cc: <stable@vger.kernel.org> [4.1+]
Reported-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>