iwlwifi: pcie: allow sending pre-built A-MSDUs

iwlwifi: pcie: allow sending pre-built A-MSDUs

In case of A-MSDUs, the trans layer is taking care of building
the subframes (out of the given skb), according to the given gso_size.

However, in some testing flows, we want to build the whole A-MSDU
frame in a different place (e.g. userspace), and ask the driver
to send it as-is.

In case of gso_size==0, simply treat the frame as normal-frame,
although the A-MSDU flag is set.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>