phy: rockchip-usb: move per-phy init into a separate function

This unclutters the loop in probe a lot and makes current (and future)
error handling easier to read.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>