iwlwifi: pcie: use iwl_tx_cmd_gen2 and not iwl_tx_cmd

When building AMSDU for gen2, code uses iwl_tx_cmd. The only
updated field is len, which is in the same location, so it
is not a bug. However, it is a bit confusing and error prone,
so change it.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>