media: bcm2835-unicam: Clear clock state when stopping streaming

Commit 65e08c465020d4c5b51afb452efc2246d80fd66f failed to clear the
clock state when the device stopped streaming. Fix this, as it might
again cause the same problems when doing an unprepare.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>