net: phy: don't double-read link status register if link is up

The link status register latches link-down events. Therefore, if link
is reported as being up, there's no need for a second read.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>