media: v4l2-common.h: kerneldoc: correctly format return values

Building the kerneldoc resulted in two errors:

Documentation/media/driver-api/v4l2-common:6: ./include/media/v4l2-common.h:566: ERROR: Unexpected indentation.
Documentation/media/driver-api/v4l2-common:6: ./include/media/v4l2-common.h:567: WARNING: Block quote ends without a blank line; unexpected unindent.

Format v4l2_link_freq_to_bitmap according to the kerneldoc standard.
The v4l2_fill_pixfmt_mp function also had incorrect return value
formatting, although that didn't report an error/warning, but it looked
ugly in the generated documentation. So fix that one as well.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: a68e88e2 ("media: v4l: Add a helper for setting up link-frequencies control")
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>