Staging: iio: adc: Fix warning prefer kmalloc_array over kmalloc

This patch fixes the checkpatch warning:
WARNING: Prefer kmalloc_array over kmalloc with multiply

kmalloc_array is preffered for allocating for arrays.

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>