Support one-shot MSI on 5787.

This one-shot MSI idea is credited to David Miller. In this mode, MSI
disables itself automatically after it is generated, saving the driver
a register access to disable it for NAPI.
Signed-off-by: NMichael Chan <mchan@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>