meson saradc: fix clock divider mask length

meson saradc: fix clock divider mask length

According to the datasheets of supported meson SoCs length of ADC_CLK_DIV
field is 6-bit. Although all supported SoCs have the register
with that field documented later SoCs use external clock rather than
ADC internal clock so this patch affects only meson8 family (S8* SoCs).

Fixes: 3adbf3427330 ("iio: adc: add a driver for the SAR ADC found in Amlogic Meson SoCs")
Signed-off-by: George Stark <GNStark@sberdevices.ru>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20230606165357.42417-1-gnstark@sberdevices.ru
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>