drivers: phy: Set phy->dev to NULL when generic_phy_get_by_index() fails

phy->dev need to be set to NULL in case of generic_phy_get_by_index()
fails. Then phy->dev can be used to check if the phy is valid

Reported-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>