Staging: iio: Documentation: Added check for sscanf return value.

The following patch fixes the following checkpatch.pl warning by adding
a check for sscanf return value.
WARNING: unchecked sscanf return value.

Signed-off-by: Heena Sirwani <heenasirwani@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>