adc: Cleanup main Kconfig

This patch addresses the following issues:
* Aligns the Kconfig code style with Zephyr projects requirements.
* Removes redundant "depends on" from Kconfig.

Change-Id: I46a156581cdf79d0ba8f0030ce7b595469db1bcb
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>