net: phy: xgmiitorgmii: Support generic PHY status read

Some PHY drivers like the generic one do not provide a read_status
callback on their own but rely on genphy_read_status being called
directly.

With the current code, this results in a NULL function pointer call.
Call genphy_read_status instead when there is no specific callback.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>