usb: dwc3: document async_callbacks field

Avoid a kernel-doc warning by documenting it:
	drivers/usb/dwc3/core.h:1328: warning: Function parameter or member 'async_callbacks' not described in 'dwc3'

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/2c3de5935934baec097286f525ce4beff0a31ec0.1656409369.git.mchehab@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>