[Snek] [keith-packard/snek] 5dbfa5: chips/samd21: Try to stabilize ADC

[Keith Packard]

  Branch: refs/heads/master
  Home:   https://github.com/keith-packard/snek
  Commit: 5dbfa54f02e8a52991f512cea9b240c1306bde7f
      https://github.com/keith-packard/snek/commit/5dbfa54f02e8a52991f512cea9b240c1306bde7f
  Author: Keith Packard <keithp at keithp.com>
  Date:   2020-02-23 (Sun, 23 Feb 2020)

  Changed paths:
    M chips/samd21/ao-adc-samd21.c

  Log Message:
  -----------
  chips/samd21: Try to stabilize ADC

Use lots more ao_adc_sync calls.
Do two conversions, discard the first.

Signed-off-by: Keith Packard <keithp at keithp.com>