iio: adc: at91_adc: correct default shtim value

When sample_hold_time is zero (this is the case when DT is not used or if
atmel,adc-sample-hold-time is omitted), then the calculated shtim is large.

Make that 0, which is the default for that register and the ADC will then use a
sane value of 2/ADCCLK or 1/ADCCLK depending on the version.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>