vc04_services: bcm2835_codec: Ignore READ_ONLY ctrls in s_ctrl

In adding the MPEG2/MPEG4/H264 level and profile controls to
the decoder, they weren't declared as read-only, nor handlers
added to bcm2835_codec_s_ctrl. That resulted in an error message
"Invalid control" being logged every time v4l2_ctrl_handler_setup
was called from bcm2835_codec_create_component.

Define those controls as read only, and exit early from s_ctrl
on read only controls.

Fixes: "media: bcm2835-v4l2-codec: Add profile & level ctrls to decode"
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>