net: phy: Do not check Link status when loopback is enabled

While running stmmac selftests I found that in my 1G setup some tests
were failling when running with PHY loopback enabled.

It looks like when loopback is enabled the PHY will report that Link is
down even though there is a valid connection.

As in loopback mode the data will not be sent anywhere we can bypass the
logic of checking if Link is valid thus saving unecessary reads.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>