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

drivers/usb/dwc3/core.h

@@ -1087,6 +1087,8 @@ struct dwc3_scratchpad_array {
  * @dis_u1_entry_quirk: set if link entering into U1 state needs to be disabled.
  * @dis_u2_entry_quirk: set if link entering into U2 state needs to be disabled.
  * @dis_rxdet_inp3_quirk: set if we disable Rx.Detect in P3
+ * @async_callbacks: if set, indicate that async callbacks will be used.
+ *
  * @dis_u2_freeclk_exists_quirk : set if we clear u2_freeclk_exists
  *			in GUSB2PHYCFG, specify that USB2 PHY doesn't
  *			provide a free-running PHY clock.