iio: temperature: ltc2983: make bulk write buffer DMA-safe

regmap_bulk_write() does not guarantee implicit DMA-safety,
even though the current implementation duplicates the given
buffer. Do not rely on it.

Fixes: f110f318

 ("iio: temperature: Add support for LTC2983")
Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Link: https://lore.kernel.org/r/20221103130041.2153295-2-demonsingur@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>