phy: meson-gxl-usb2: add set_mode callback

phy: meson-gxl-usb2: add set_mode callback

Implement set_mode callback by calling the current public
function, use a temporary function name that will be removed
when the public phy_meson_gxl_usb2_set_mode is finally removed
in a following change.

Reviewed-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20240620-u-boot-usb-gxl-phy-set-mode-v2-1-b81c027bc02c@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>