lavc/v4l2_context: Change the type of the ioctl cmd to uint32_t.

Fixes a warning on recent Linux:
libavcodec/v4l2_context.c: In function 'ff_v4l2_context_set_status':
libavcodec/v4l2_context.c:496:26: warning: comparison is always false due to limited range of data type
