phy: Fix possible NULL pointer deference

It is possible that users of generic_phy_*() APIs may pass a valid
struct phy pointer but phy->dev can be NULL, leading to NULL pointer
deference in phy_dev_ops().

So call generic_phy_valid() to verify that phy and phy->dev are both
valid.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>