drm/sun4i: hdmi ddc clk: Fix size of m divider

m divider in DDC clock register is 4 bits wide. Fix that.

Fixes: 9c568101 ("drm/sun4i: Add HDMI support")
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200413095457.1176754-1-jernej.skrabec@siol.net