iwlwifi: mvm/pcie: adjust A-MSDU tx_cmd length in PCIe

Instead of setting the tx_cmd length in the mvm code, which is
complicated by the fact that DQA may want to temporarily store
the SKB on the side, adjust the length in the PCIe code which
also knows about this since it's responsible for duplicating
all those headers that are account for in this code.

As the PCIe code already relies on the tx_cmd->len field, this
doesn't really introduce any new dependencies.

To make this possible we need to move the memcpy() of the TX
command until after it was updated.

This does even simplify the code though, since the PCIe code
already does a lot of manipulations to build A-MSDUs correctly
and changing the length becomes a simple operation to see how
much was added/removed, rather than predicting it.

Fixes: 24afba76 ("iwlwifi: mvm: support bss dynamic alloc/dealloc of queues")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>