iio: Add __printf() attributes to various allocation functions

A partial set of these was added to IIO a long time back.
This fills in some gaps in coverage highlighted by building
with W=1

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200913132115.800131-3-jic23@kernel.org