alx: add ability to allocate and free alx_napi structures

Add new functions to allocate and free the alx_napi structures and use them
in __alx_open and __alx_stop. We only allocate one of these structures for
now, as the rest of the driver is not yet ready for multiple queues.

We switch over the setup of the interrupt mask and the call to netif_napi_add
to the new function because we must adjust these later on a per queue basis.

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>