iio: buffer: Use dedicated variable in iio_buffers_alloc_sysfs_and_mask()

Use dedicated variable for index in the loop in the
iio_buffers_alloc_sysfs_and_mask(). This will make code cleaner and
less error prone as proved by previous changes done in this function.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211013094923.2473-3-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>