alx: prepare resource allocation for multi queue support

Allocate, initialise and free alx_tx_queue structs based on the number of
alx_napi structures. Also increase the size of the descriptor memory based
on the number of tx queues in use.

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>