lan78xx: Check device ready bit (PMT_CTL_READY_) after reset the PHY

Check device ready bit (PMT_CTL_READY_) after reset the PHY.
Device may not be ready even if PHY_RST_ is cleared depends on configuration.

Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>