Fix XHCI driver for devices which have more than 15 physical root HUB

ports. The current bitmap array was too small to hold more than 16
bits and would at some point toggle the context size, which then would
trigger an enumeration fault and cause a fallback to the EHCI
companion controller, if any.

MFC after:	3 days

sys/dev/usb/controller/xhci.h

@@ -493,7 +493,8 @@ struct xhci_softc {
 	uint8_t			sc_noscratch;
 	/* root HUB device configuration */
 	uint8_t			sc_conf;
-	uint8_t			sc_hub_idata[2];
+	/* root HUB port event bitmap, max 256 ports */
+	uint8_t			sc_hub_idata[32];
 
 	/* size of context */
 	uint8_t			sc_ctx_is_64_byte;