ITE: drivers/adc: fix the sampling flow of ADC

The ADC sampling of it8xxx2 needs to read each channel in sequence,
so it needs to wait for an interrupt to read data in the loop
through k_sem_take().

In test_adc.c, k_timer_start() is used in the interval test, so we
need to use polling wait instead of k_sem_take() to wait, otherwise
it will cause kernel panic.

k_is_in_isr() can determine whether to use polling or k_sem_take()
at present.

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