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nusb/usbd: retry opendevtata() a few times on attach (work around mnt-reform trackball)

this is a known and already fixed issue with version the following version
of the mnt-reform trackball firmware:

https://source.mnt.re/reform/reform/-/commit/55ca5bf848b57cdf6e7243fc1018411f6c8ab9b5

--- a/sys/src/cmd/nusb/usbd/hub.c

+++ b/sys/src/cmd/nusb/usbd/hub.c

@@ -355,6 +355,7 @@

 	char *sp;

 	int mp;

 	int nr;

+	int i;

 	d = h->dev;

 	pp = &h->port[p];

@@ -421,9 +422,13 @@

 	nd->isusb3 = h->dev->isusb3;

 	if(usbdebug > 2)

 		devctl(nd, "debug 1");

-	if(opendevdata(nd, ORDWR) < 0){

+	for(i=0;; i++){

+		if(opendevdata(nd, ORDWR) >= 0)

+			break;

 		fprint(2, "%s: %s: opendevdata: %r\n", argv0, nd->dir);

-		goto Fail;

+		if(i >= 4)

+			goto Fail;

+		sleep(500);

 	}

 	if(usbcmd(nd, Rh2d|Rstd|Rdev, Rsetaddress, nd->id, 0, nil, 0) < 0){

 		dprint(2, "%s: %s: port %d: setaddress: %r\n", argv0, d->dir, p);