drivers: adc: saadc: Disable burst mode on unused channels

Burst mode enabled on an unused channel causes a freeze during a
conversion consisting of several channels (not including the one with
burst mode enabled).

Discovered on nRF52832 (nRF52-DK) using the following approach:
  channels 0-3 are used for application purposes as a sequence
  channel 4 is used for battery measurements with oversampling

After few successful conversions the sequence (channels 0-3) freezes the
thread while waiting for semaphore to end the conversion.

Signed-off-by: Maksim Salau <maksim.salau@gmail.com>