zinger: fix double ADC interrupt bug
zinger: fix double ADC interrupt bug

The ADC interrupt does not clear the NVIC pending register. This
can cause the interrupt to fire more than once for a given
interrupt.

BUG=none
BRANCH=samus
TEST=Send hard reset from samus to zinger using "pd 1 hard" on
PD MCU console. This causes zinger to cut its output voltage
and go into voltage discharging mode. When voltage discharge is
complete, we get an ADC interrupt and switch back to current
monitoring. Before this CL, sometimes (1 out of 20) times the
ADC interrupt will fire twice, causing an OCP to be detected.
With this CL, we never see the double fire.

Change-Id: I91397a04773d04e263bc80a698c8799342b80a2e
Signed-off-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/223381
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
(cherry picked from commit 27d60bb83eae453955ae55235ea31d7aa2d39a3f)
Reviewed-on: https://chromium-review.googlesource.com/223739

thehobn/ec,thehobn/ec,thehobn/ec,thehobn/ec