iwlwifi: pcie: don't pad AMSDU packets

When we TX AMSDU, we shouldn't pad the packet. In the past,
we were building AMSDU only in transport layer, and gen2
functions are built based on this. However, now that op mode
may build AMSDUs, we need to take care of padding also in
gen2 "non-pcie-amsdu" path.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>