phy: core: Fix of_phy_provider_lookup to return PHY provider for sub node

Fixed of_phy_provider_lookup to return 'phy_provider' if _of_phy_get
passes the node pointer of the sub-node of phy provider node. This is
needed when phy provider implements multiple PHYs and each PHY is
modelled as the sub-node of PHY provider device node.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Lee Jones <lee.jones@linaro.org>