media: coda: add coda_frame_type_char helper

Add a function to translate from V4L2 buffer flags to 'I'/'P'/'B'
characters for debug output.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>