staging:iio:buffer: pull computation of scan length into a utility function.

Principal reason is to make later patches more coherent and easier to review
but this set in itself separates a logical entity out nicely wihin the code.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>