pasemi_mac: Don't enable RX/TX without a link (if possible)

Don't enable RX/TX of packets until we have a link, since there's a chance
we'll just get RX frame errors, etc.

The case where we don't have a PHY we can't do much about: Just enable
it and deal with errors as they come in.
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>