iio: Add helper function for calculating scan index storage size

We have the same code for computing the scan index storage size in bytes
all over the place. Factor this out into helper functions.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>