staging:iio: Remove unused variable

Fixes the following warning:
	drivers/staging/iio/industrialio-buffer.c: In function ‘iio_scan_mask_query’:
	drivers/staging/iio/industrialio-buffer.c:620: warning: unused variable ‘mask’
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/industrialio-buffer.c

@@ -617,8 +617,6 @@ EXPORT_SYMBOL_GPL(iio_scan_mask_set);
 int iio_scan_mask_query(struct iio_dev *indio_dev,
 			struct iio_buffer *buffer, int bit)
 {
-	long *mask;
-
 	if (bit > indio_dev->masklength)
 		return -EINVAL;