alx: prepare interrupt functions for multiple queues

Extend the interrupt bringup code and the interrupt handler for msi-x
interrupts in order to handle multiple queues.

We must change the poll function because with multiple queues it is possible
that an alx_napi structure has only a tx or only a rx queue pointer.

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>