[media] coda: avoid calling SEQ_END twice

Allow coda_seq_end_work to be called multiple times, move the setting
of ctx->initialized from coda_start/stop_streaming() into
coda_start_encoding/decoding and coda_seq_end_work, respectively,
and skip the SEQ_END command in coda_seq_end_work if the context is
already deinitialized before.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>