[realtek] Update link state when device is opened

[realtek] Update link state when device is opened

The link state is currently set at probe time, and updated only when
the device is polled.  This results in the user seeing a misleading
stale "Link: down" message, if autonegotiation did not complete within
the short timespan of the probe routine.

Fix by updating the link state when the device is opened, so that the
message that ends up being displayed to the user reflects the real
link state at device open time.

Signed-off-by: Michael Brown <mcb30@ipxe.org>