tests: driver: adc: change the sampling delay

patch fix the issue of unable to read from adc by setting
appropriate sampling delay.

The sampling delay should be set to resolution of adc + 2.
In adc qmsi driver the sampling delay received form application
is subtracted from resolution of adc + 2 , if the resultant value
is less than zero then zero is set as sampling delay. So in the
adc_simple application when 12 is passed as sampling delay driver
sets it as zero. So 26 is the minimum sampling delay should be
set in application for a adc with resolution of 12. The reason
to choose 30 was to maintain same value of sampling
delay between adc_simple and adc_api application.

Signed-off-by: Savinay Dharmappa <savinay.dharmappa@intel.com>