iio: core: Add support for writing 64 bit attrs

Prior to this patch it was only possible to read 64 bit integers.

Signed-off-by: Sam Winchenbach <swinchenbach@arka.org>
Link: https://patch.msgid.link/20250328174831.227202-6-sam.winchenbach@framepointer.org


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