usb: phy: ab8500-usb: move phy tuning values on separate functions

Move each chip's PHY tuning value set code to a separate function to
improve code readability.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>