[media] pwc: do not decompress the image unless the state is DONE

There is no point in trying to decompress a captured frame unless
the buffer state is OK. It won't be used in any other state, and
in fact the contents of the buffer might well be corrupt.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>