ixgbevf: Synch out of tree and in tree mailbox interrupt handlers

The out of tree driver and the in kernel driver should use the same
interrupt handling logic for mailbox interrupts.  The difference in
the handlers was causing dissimilar behavior between the two drivers
complicating debug and trouble shooting.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>