usbdebug: Probe for gadget only once

The first stage attempting to initialise usbdebug
gadget will leave it marked as non-present if none
is detected. This allows further stages to bypass
usbdebug init sequence.

Change-Id: I1491d7fab3c89f210fb03b32481f697bc7a1d1e6
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/30622
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/drivers/usb/ehci_debug.h

@@ -31,6 +31,7 @@ void ehci_debug_select_port(unsigned int port);
 #define DBGP_EP_VALID		(1<<0)
 #define DBGP_EP_ENABLED		(1<<1)
 #define DBGP_EP_BUSY		(1<<2)
+#define DBGP_EP_NOT_PRESENT	(1<<3)
 #define DBGP_EP_STATMASK	(DBGP_EP_VALID | DBGP_EP_ENABLED)
 
 #define DBGP_MAX_ENDPOINTS	4