USB: UDC: Implement udc_async_callbacks in net2280

This patch adds a udc_async_callbacks handler to the net2280 UDC
driver, which will prevent a theoretical race during gadget unbinding.

The net2280 driver is sufficiently complicated that I didn't want to
mess around with IRQ settings.  Instead, the patch simply adds a new
flag to control async callbacks, and checks the flag before issuing
any of them.

Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20210520202200.GE1216852@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>