serial: 8250: replace hardcoded 0xbf with #define

Makes it easier to find all occurences requesting CONF_MODE_B.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>