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

If probe errors out after request_irq(), its error path
does not explicitly cancel the delayed work, which may
have been scheduled by the interrupt handler.

This means the delayed work may still be running when
the core frees the private structure (struct xadc).
This is a potential use-after-free.

Fix by inserting cancel_delayed_work_sync() in the probe
error path.

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