[media] mtk-vcodec: fix some smatch warnings

Fix this bug:
	drivers/media/platform/mtk-vcodec/vdec_drv_if.c:38 vdec_if_init() info: ignoring unreachable code.

With is indeed a real problem that prevents the driver to work!

While here, also remove an used var, as reported by smatch:

	drivers/media/platform/mtk-vcodec/mtk_vcodec_dec_pm.c: In function 'mtk_vcodec_init_dec_pm':
	drivers/media/platform/mtk-vcodec/mtk_vcodec_dec_pm.c:29:17: warning: variable 'dev' set but not used [-Wunused-but-set-variable]
	  struct device *dev;
	                 ^~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>