sh: Improve kfr2r09 serial port setup code

This patch improves the serial port communication quality
of port YC401 on the KFR2R09 board. With this fix serial
console is fine at 115200 - up and down keys now work as
expected. Thanks to Hirohide Yamasaki for this fix.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>