iio: buffer: hw-consumer: remove redundant scan_mask flexible array

iio: buffer: hw-consumer: remove redundant scan_mask flexible array

The scan_mask flexible array member in hw_consumer_buffer duplicates
the scan_mask pointer already present in struct iio_buffer. Remove it
and allocate the bitmap directly with bitmap_zalloc(), assigning it to
buf->buffer.scan_mask. This simplifies the code and there's no need for
the flex array allocation.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>