iio: adc: at91-sama5d2_adc: fix AT91_SAMA5D2_MR_TRACKTIM_MAX

All ADC HW versions handled by this driver (SAMA5D2, SAM9X60, SAMA7G5)
have MR.TRACKTIM on 4 bits. Fix AT91_SAMA5D2_MR_TRACKTIM_MAX to reflect
this.

Fixes: 27e17719 ("iio:adc:at91_adc8xx: introduce new atmel adc driver")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220803102855.2191070-2-claudiu.beznea@microchip.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>