igb: fix handling of mailbox collisions between PF/VF

This patch changes the handling of collisions between the use of the
PF/VF sides of the mailbox.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>