iwlwifi: mvm: take care of padded packets

To ensure that the SNAP/TCP/IP headers are DW aligned, the firmware
may add 2-byte pad at the end of the mac header - after the IV, before
the SNAP.
In that case the mpdu descriptor pad bit will be turned on.
Driver should take it into consideration, and remove the padding before
passing the packet to mac80211. Do that.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>