igb: reconfigure mailbox timeout logic

This change updates the timeout logic so that it is not possible to have a
sucessful check for message and still return an error if countdown = 0.

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