qed: Use ieee mfw-mask to get ethtype in ieee-dcbx mode.

Ethtype value is being read incorrectly in ieee-dcbx mode. Use the
correct mfw mask value.

Signed-off-by: Sudarsana Reddy Kalluru <sudarsana.kalluru@qlogic.com>
Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>