iio: light: us5182d: Refactor read_raw function

A bit of refactoring for better readability.
Moved and slightly reorganized all the activity necessary for reading als
and proximity into a different function. This way the switch in read raw
becomes clearer and more compact.

Signed-off-by: Adriana Reus <adriana.reus@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>