stm32: Fix ADC watchdog bug and extend sample time
stm32: Fix ADC watchdog bug and extend sample time

This fixes a bug in ADC watchdog that sample sequence is not set before
enabling ADC watchdog. Also, current sample time isn't long enough for
some weakly driven net. This extends ADC sample time to 13.5 cycles.

BUG=chrome-os-partner:17928
TEST=Test with Toad cable and video dongle on Spring
BRANCH=spring

Change-Id: Iecdfd4aada4e974225a41144e213e92897f4797b
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/49680
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

mtk09422/chromiumos-platform-ec,akappy7/ChromeOS_EC_LED_Diagnostics,alterapraxisptyltd/chromium-ec,eatbyte/chromium-ec,coreboot/chrome-ec,longsleep/ec,fourier49/BIZ_EC,thehobn/ec,mtk09422/chromiumos-platform-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,longsleep/ec,mtk09422/chromiumos-platform-ec,fourier49/BZ_DEV_EC,coreboot/chrome-ec,longsleep/ec,longsleep/ec,alterapraxisptyltd/chromium-ec,thehobn/ec,eatbyte/chromium-ec,gelraen/cros-ec,md5555/ec,fourier49/BIZ_EC,coreboot/chrome-ec,fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,eatbyte/chromium-ec,alterapraxisptyltd/chromium-ec,coreboot/chrome-ec,gelraen/cros-ec,md5555/ec,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,md5555/ec,alterapraxisptyltd/chromium-ec,gelraen/cros-ec,thehobn/ec,eatbyte/chromium-ec,thehobn/ec,coreboot/chrome-ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,gelraen/cros-ec,mtk09422/chromiumos-platform-ec,md5555/ec