media: atomisp: fix the handling of clock number

Right now, the driver is not doing the right thing to detect
the clock like used by the sensor, at least on devices
without the gmin's EFI vars.

Add some notes at the code to explain why and skip the wrong
value provided by the _DSM table.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>