phy: rockchip-typec: Do the calibration more correctly

Calculate the calibration code as per the docs.  The docs talk about
reading and averaging the pullup and pulldown calibration codes.  They
also talk about adding in some adjustment codes.  Let's do what the
docs say.

In practice this doesn't seem to matter a whole lot.  On a device I
tested the pullup and pulldown codes were nearly the same (0x23 and
0x24) and the adjustment codes were 0.

Reviewed-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>