aio: Cleanup AIO comparator driver

This patch addresses the following issues:
* Aligns the Kconfig code style with Zephyr projects requirements.
* Removes redundant "depends on" from Kconfig.
* Adds static to the containing file scope declarations.

Change-Id: I03326a800392cffda00b47949981b7e6d119b90f
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>