drm/bridge: ti-sn65dsi86: Check the regmap return value when setting a GPIO

The ti_sn_bridge_gpio_set() got the return value of
regmap_update_bits() but didn't check it.  The function can't return
an error value, but we should at least print a warning if it didn't
work.

This fixes a compiler warning about setting "ret" but not using it.

Fixes: 27ed2b3f

 ("drm/bridge: ti-sn65dsi86: Export bridge GPIOs to Linux")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200612123003.v2.4.Ia4376fd88cdc6e8f8b43c65548458305f82f1d61@changeid