ixgbe: Exit on error case in VF message processing

Previously we would get a mailbox error and still process the message.
Instead we should exit on error.

In addition we should also be flushing the ACK of the message so that we
can guarantee that the other end is aware we have received the message
while we are processing it.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>