serial: 8250: Support XR17V35x fraction divisor

The Exar XR17V35x family of UARTs have an additional fractional divisor
register (DLD) which was not being used. Calculate and set this
register for these devices to reduce their baud rate error.

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