media: atomisp-ov2680: Turn on power only once

Add a power_on bool to track if the power is on, and make
power_up() a no-op if the power is already on.

This also removes a power_down() call from ov2680_s_config() since
that is a no-op now, this is ok because s_config() is only called
once on probe and the sensor is off at boot.

Besides avoiding to the work in power_up() multiple times this patch
is also a preparation for switching to the clk and regulator frameworks
which keep an enable count, so there we must call enable() and
disable() only once per power-cycle.

Link: https://lore.kernel.org/linux-media/20211107171549.267583-3-hdegoede@redhat.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>