ixgbevf: Fix mailbox interrupt ack bug

Remove jump to out label that was skipping over code to replace the
ack bit in the mailbox cache variable.  This was causing driver mailbox
routines that poll for the PF to ack a message to time out which would
in turn cause all other mailbox messages to fail.  Also replace the
mailbox cache variable message indication bit when a message is found
so that other functions that choose to poll for a mailbox message from
the PF won't miss it.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Robert E Garrett <robertX.e.garrett@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>