iio: adc: xilinx: fix potential use-after-free on remove

[ Upstream commit 62039b6aef63380ba7a37c113bbaeee8a55c5342 ]

When cancel_delayed_work() returns, the delayed work may still
be running. This means that the core could potentially free
the private structure (struct xadc) while the delayed work
is still using it. This is a potential use-after-free.

Fix by calling cancel_delayed_work_sync(), which waits for
any residual work to finish before returning.

Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>