media: v4l2-flash: Add sanity checks for flash and indicator controls

The V4L2 flash API supports combinations of indicator and flash LEDs. Due
to this, there's a fair amount of code that deals with all the possible
options and just reading one part of the file doesn't really tell which
combinations are really possible.

Make the checks more explicit to keep static analysers happy and to make
the code more resilient to future mishaps.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>