media: smiapp: Rely on runtime PM

Instead of relying on a mix of runtime PM and the s_power() callback, drop
the s_power() callback altogether and use runtime PM solely.

As device access is required during device power-on and power-off
sequences, runtime PM alone cannot tell whether the device is available.
Thus the "active" field is introduced in struct smiapp_sensor to tell
whether it is safe to write to the device.

Consequently there is no need to power on the device whenever a file
handle is open. This functionality is removed as well. The user may still
control the device power management through sysfs. Autosuspend remains
enabled, with 1 s delay.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>