alx: switch to per queue data structures

Remove the tx and rx queue structures from the alx_priv structure and switch
everything over to the queue pointers in the alx_napi structure.

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>