iio: adc: ltc2497: Fix alignment for DMA safety

iio: adc: ltc2497: Fix alignment for DMA safety

____cacheline_aligned is an insufficient guarantee for non-coherent DMA
on platforms with 128 byte cachelines above L1.  Switch to the updated
IIO_DMA_MINALIGN definition.

Update the comment to include 'may'.

Fixes: bc82222fcca1 ("iio:adc: Driver for Linear Technology LTC2497 ADC")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-23-jic23@kernel.org