phy: core: Get a refcount to phy in devm_of_phy_get_by_index()

On driver detach, devm_phy_release() will put a refcount to
the phy, so gets a refconut to it before return.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>