Staging: iio: adc: Add missing blank line

This fixes following checkpatch.pl warningi:

WARNING: Missing a blank line after declarations

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>