HID: mcp-2221: prevent UAF in delayed work

In the Linux kernel, the following vulnerability has been resolved:

HID: mcp-2221: prevent UAF in delayed work

If the device is plugged/unplugged without giving time for mcp_init_work() to complete, we might kick in the devm free code path and thus have unavailable struct mcp_2221 while in delayed work.

Canceling the delayed_work item is enough to solve the issue, because cancel_delayed_work_sync will prevent the work item to requeue itself.