ps8xxx: add forward declare usb_mux
ps8xxx: add forward declare usb_mux

This adds a forward declaration for struct usb_mux used in a function
prototype.

BRANCH=none
BUG=none
TEST=buildall passes

Change-Id: I647c11b86dbed180bdd025063468ecdb651fd8a7
Signed-off-by: Caveh Jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2654738
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Commit-Queue: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec