serial: sh-sci: Preserve SCIFA/SCIFB bit rate config for serial console

SCIFA and SCIFB have additional bit rate config bits in the Serial Mode
Register.  Don't touch them when using the port as a serial console, as
we rely on the boot loader to have configured the serial port config.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>