serial: 8250: Preserve DLD[7:4] for PORT_XR17V35X

The upper four bits of the XR17V35x fractional divisor register (DLD)
control general chip function (RS-485 direction pin polarity, multidrop
mode, XON/XOFF parity check, and fast IR mode). Don't allow these bits
to be clobbered when setting the baudrate.

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>