ITE: drviers/adc: Adjust the wait time for acquiring a semaphore

After the ADC interrupt is enabled, the interrupt will be triggered
and the ADC channel valid data will be read, so there is no time
limit here.
Refer to the timeout is also set to K_FOREVER in the function of
adc_context_wait_for_completion().

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>