As kernel-doc doesn't support documenting #define values,
and using enum makes easier to identify where the values
are used, convert V4L2_MBUS_FRAME_DESC_FL_* to enum, and
use BIT() macro.

While here, fix the description at v4l2_mbus_frame_desc_entry,
in order to match what's described for
V4L2_MBUS_FRAME_DESC_FL_LEN_MAX.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>