altos/lpc: Stop using burst mode for LPC ADC

Burst mode doesn't stop after one round of conversions, so we end up
getting incorrect values in whatever the last conversion register is.

Just use single conversions and take an interrupt per channel.

Also, slow down the ADC so that our values are more stable -- just
need to make sure we get the whole conversion sequence done 100 times
a second.

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