mlxsw: convert to pci_alloc_irq_vectors

Trivial conversion as only one vector is supported, but at least we
lose the useless msix_entry member in the per-device structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>