iio: xilinx-xadc-core: Use local variable in xadc_read_raw

Minor cleanup: bit is already defined as chan->scan_type.realbits,
use bit when needed.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Link: https://lore.kernel.org/r/20211104082413.3681212-13-gwendal@chromium.org


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