clock-8916: Change debug_mux_priv to static

debug_mux_priv should not be exposed outside of clock-8916.c. Add
static to fix multiple definition errors.

Change-Id: I6dcdb16d695d1f43fdf8567a41ec80043a1a1509
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>