USB: UDC: Implement udc_async_callbacks in dummy-hcd

This patch adds a udc_async_callbacks handler to the dummy-hcd UDC
driver, which will prevent a theoretical race during gadget unbinding.

The implementation is simple, since dummy-hcd already has a flag to
keep track of whether emulated IRQs are enabled.  All the handler has
to do is store the enable value in the flag, and avoid setting the
flag prematurely.

Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20210520202152.GD1216852@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>