net: phy: realtek: clear 1000Base-T lpa if link is down

Only read 1000Base-T link partner advertisement if autonegotiation has
completed and otherwise 1000Base-T link partner advertisement bits.

This fixes bogus 1000Base-T link partner advertisement after link goes
down (eg. by disconnecting the wire).
Fixes: 5cb409b3 ("net: phy: realtek: clear 1000Base-T link partner advertisement")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>