media: atomisp: split add from find subdev

There's only one place where a subdev can be added: when the
sensor driver registers it. Trying to do it elsewhere will
cause problems, as the detection code needs to access the
I2C bus in order to probe some things.

Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>