[media] coda: rename prescan_failed to hold and stop stream after timeout

Rename the per-context prescan_failed variable to hold, as this is what the
flag  does: it temporarily keeps the coda from running until new data is fed
into the bitstream buffer or stop_streaming is called on the input side.
A prescan failure on i.MX5 is one possible reason to enter this state, another
one is a picture run timeout on i.MX6.

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