iio: buffer-dmaengine: use %zu specifier for sprintf(align)

The 'size_t' type behaves differently on 64-bit architectures, and causes
compiler a warning of the sort "format '%u' expects argument of type
'unsigned int', but argument 3 has type 'size_t {aka long unsigned int}'".

This change adds the correct specifier for the 'align' field.

Fixes: 4538c185 ("iio: buffer-dmaengine: Report buffer length requirements")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>