alx: refactor msi enablement and disablement

Introduce a new flag field for the advanced interrupt capatibilities and add
new functions to enable and disable msi interrupts. These functions will be
extended later to cover msi-x interrupts.

We enable msi interrupts earlier in alx_init_intr because with msi-x and multi
queue support the number of queues must be set before we allocate resources for
the rx and tx paths.

Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>