pch_gbe: minor: report the actual error on MTU change

If we can't _up() after changing the MTU, report the actual error instead
of -ENOMEM. It can be really misleading cause pch_gbe is usually used in
scenarios where the memory amount is really small, and thus hiding the
real cause.

Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>