net: zynq: Use generic_phy_valid() helper

The documentation for struct phy state that "The content of the
structure is managed solely by the PHY API and PHY drivers".

Change to use the generic_phy_valid() helper to check if phy is valid.

Fixes: 10c50b1f ("net: zynq: Add support for PHY configuration in SGMII mode")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>