i3c: master: make i3c_bus_set_mode static

i3c_bus_set_mode function is only used in master.c.
Make it static to avoid warning when compiling with W=1.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>