drivers/tty/serial/8250/8250_core.c: remove CONFIG_HUB6

This file local CPP identifier is not referenced anywhere else, so we
can safely remove it.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>