usb: prevent double opening

This could happen in gintctl's USB tracer and obviously cause all sorts
of problems.

include/gint/usb.h

@@ -39,6 +39,8 @@ enum {
 	USB_OPEN_MISSING_DATA,
 	/* Invalid parameters: bad endpoint numbers, bad buffer sizes... */
 	USB_OPEN_INVALID_PARAMS,
+	/* USB interfaces are already opened */
+	USB_OPEN_ALREADY_OPEN,
 
 	/* This pipe is busy (returned by usb_write_async()) */
 	USB_WRITE_BUSY,

src/usb/usb.c

@@ -121,6 +121,9 @@ static void hpoweroff(void)
 
 int usb_open(usb_interface_t const **interfaces, gint_call_t callback)
 {
+	if(usb_open_status)
+		return USB_OPEN_ALREADY_OPEN;
+
 	/* TODO: Check whether the calculator can host devices (probably no) */
 	bool host = false;