usb: core: reduce power-on-good delay time of root hub

Return the exactly delay time given by root hub descriptor,
this helps to reduce resume time etc.

Due to the root hub descriptor is usually provided by the host
controller driver, if there is compatibility for a root hub,
we can fix it easily without affect other root hub
Acked-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NChunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1618017645-12259-1-git-send-email-chunfeng.yun@mediatek.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/hub.h

@@ -148,7 +148,9 @@ static inline unsigned hub_power_on_good_delay(struct usb_hub *hub)
 {
 	unsigned delay = hub->descriptor->bPwrOn2PwrGood * 2;
 
-	/* Wait at least 100 msec for power to become stable */
+	if (!hub->hdev->parent)	/* root hub */
+		return delay;
+	else /* Wait at least 100 msec for power to become stable */
 		return max(delay, 100U);
 }