adc: explicitly cast (const void *) to (const adc_driver_api *)
adc: explicitly cast (const void *) to (const adc_driver_api *)

Without this patch GCC 8.3 will error during a C++ build.

Signed-off-by: Andrew Fernandes <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@fernandes.org>

GiulianoFranchetto/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr