[media] media: platform: pxa_camera: change stop_streaming semantics

Instead of the legacy behavior where it was required to wait for all
video buffers to be finished by the hardware, use a cancel like strategy
: as soon as the stop_streaming() call is done, abort all DMA transfers,
report the already buffers as failed and return.

This makes stop_streaming() more a "cancel capture" than a "wait for end
of capture" semantic.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>