[media] coda: clear RET_DEC_PIC_SUCCESS flag in prepare_decode

To make sure a set RET_DEC_PIC_SUCCESS flag is not a leftover from
a previous successful run, clear it in prepare_decode.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>