V4L/DVB (5891): zr36067: Turn off raw capture properly

When raw capture was turned off, the current capturing frame (v4l_grab_frame)
wasn't reset to NO_GRAB_ACTIVE.  If capture was turned back on, the driver
would think this frame was currently being captured, and wait for it to
complete before starting a new frame.  The hardware on the other hand would
not be actively capturing a frame.  The result was the driver would wait
forever for v4l_grab_frame to be captured.

Some calls to zr36057_set_memgrab(0) were missing spin-locks, which have been
added.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Acked-by: Ronald S. Bultje <rbultje@ronald.bitfreak.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>