iio: adc: xilinx: prevent touching unclocked h/w on remove

In remove, the clock is disabled before canceling the
delayed work. This means that the delayed work may be
touching unclocked hardware.

Fix by disabling the clock after the delayed work is
fully canceled. This is consistent with the probe error
path order.

Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>