be2net: Use new F/W mailbox cmd to manipulate interrupts.

This is needed as the earlier method of manipulating this register via PCI
Config space is disallowed by certain Hypervisors.

Signed-off-by: Somnath Kotur <somnath.kotur@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>