Fix for phy_disconnect bug

Disconnect the phy device only if there is a PHY connected.
If the phy_mask is zero, the phydev pointer is null, therefore
check the phydevice before disconnection.
Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>