[media] smiapp: Fix power count handling

The sensor may be powered by either one of its sub-devices being accessed
from the user space (an open file handle) or by its s_power() op being
called with non-zero on argument. The driver counts the users and if any
reason to keep the device powered exists it will be powered.

However, a faulty condition was used in recognising the need to power off
the sensor, leading it to be powered off every time any of its uses went
away.

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