staging:iio:dac: Fix kcalloc parameters swapped

The first parameter should be "number of elements" and the second parameter
should be "element size".

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>