tools: iio: Add ARRAY_SIZE macro

Calculation of the length of an array can be done with the ARRAY_SIZE
macro to make code more abstract and remove the associated
checkpatch.pl warning.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>