drm/bridge: sn65dsi86: ti_sn65dsi86_read_u16() __maybe_unused

drm/bridge: sn65dsi86: ti_sn65dsi86_read_u16() __maybe_unused

When built without CONFIG_PWM there are no references to
ti_sn65dsi86_read_u16(), avoid the W=1 build warning by marking the
function as __maybe_unused.

__maybe_unused is used insted of a #ifdef guard as it looks slighly
cleaner and it avoids issues if in the future other permutations of the
config options would use the function.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 524e54a9f780 ("drm/bridge: ti-sn65dsi86: Implement the pwm_chip")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211028163548.273736-1-bjorn.andersson@linaro.org
Signed-off-by: Robert Foss <robert.foss@linaro.org>