USB: mos7840: fix null-pointer dereference

Fix null-pointer dereference on error path.

Cc: stable <stable@kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/mos7840.c

@@ -730,7 +730,6 @@ static void mos7840_bulk_in_callback(struct urb *urb)
 	mos7840_port = urb->context;
 	if (!mos7840_port) {
 		dbg("%s", "NULL mos7840_port pointer");
-		mos7840_port->read_urb_busy = false;
 		return;
 	}