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>