alx: prepare tx path for multi queue support

This patch prepares the tx path to send data on multiple tx queues. It
introduces per queue register adresses and uses them in the alx_tx_queue
structs.

There are new helper functions for the queue mapping in the tx path.

Based on the downstream driver at github.com/qca/alx

Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>