igb: update tx DMA mapping error handling

This updates the tx DMA mapping error handling code to
resemble e1000e/ixgbe.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>