phy: core: Fix error checking in (devm_)phy_optional_get

Don't pass valid pointer to PTR_ERR, use PTR_ERR(phy) only when
IS_ERR(phy) is true.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>