alx: enable multiple tx queues

Enable multiple tx queues by default based on the number of online cpus. The
hardware supports up to four tx queues.

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>