[media] v4l: atmel-isi: Defer clock (un)preparation to enable/disable time

The PCLK and MCK clocks are prepared and unprepared at probe and remove
time. Clock (un)preparation isn't needed before enabling/disabling the
clocks, and the enable/disable operation happen in non-atomic context.
We can thus defer (un)preparation to enable/disable time.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>