usb: dwc3: meson-g12a: drop usb.h and make dwc3_meson_g12a_force_mode static

Drop this useless usb.h and now make dwc3_meson_g12a_force_mode
static since only used in the dwc3-meson-g12a.c file.

Reviewed-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20240620-u-boot-usb-gxl-phy-set-mode-v2-5-b81c027bc02c@linaro.org


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>