drm/mediatek: mtk_hdmi_v2: Fix return type of mtk_hdmi_v2_tmds_char_rate_valid()

drm/mediatek: mtk_hdmi_v2: Fix return type of mtk_hdmi_v2_tmds_char_rate_valid()

When building with -Wincompatible-function-pointer-types-strict, a
warning designed to catch kernel control flow integrity (kCFI) issues at
build time, there is an instance in the new HDMI v2 drm/mediatek code:

  drivers/gpu/drm/mediatek/mtk_hdmi_v2.c:1331:31: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(const struct drm_bridge *, const struct drm_display_mode *, unsigned long long)' with an expression of type 'int (const struct drm_bridge *, const struct drm_display_mode *, unsigned long long)' [-Werror,-Wincompatible-function-pointer-types-strict]
   1331 |         .hdmi_tmds_char_rate_valid = mtk_hdmi_v2_hdmi_tmds_char_rate_valid,
        |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While 'int' and 'enum drm_mode_status' are ABI compatible, hence no
regular warning from -Wincompatible-function-pointer-types, the mismatch
will trigger a kCFI violation when mtk_hdmi_v2_tmds_char_rate_valid() is
called indirectly.

Update the return type of mtk_hdmi_v2_tmds_char_rate_valid() to be 'enum
drm_mode_status' to clear up the warning and kCFI violation.

Fixes: 8d0f79886273 ("drm/mediatek: Introduce HDMI/DDC v2 for MT8195/MT8188")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20251125-drm-mediatek-hdmi-v2-wifpts-v1-1-a6c7582cf69a@kernel.org/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>