samples: drivers: adc: minor fixes and cleanups
samples: drivers: adc: minor fixes and cleanups

- Improve include list, so that file is self-contained
- Don't use an array type for the samples buffer, as its length is 1
- Improve some error messages
- Use PRId16/32 for printing fixed-width signed integers

Signed-off-by: Gerard Marull-Paretas <bedb45f43a286e49f4dcda039a436b216ae61ef3@nordicsemi.no>

finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr