tty: 8250, drop unused members from struct old_serial_port

hub6 and irqflags from struct old_serial_port are nowhere set. Drop
them from the structure and replace the reads by zeros.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>