serial: sh-sci: Use dev_dbg() to log an error message

Since the driver cannot return from overrun error if characters
are output during overrun process, use dev_dbg() instead of
dev_notice() to log the error message of overrun in syslog.

Based on a patch by Hisashi Nakamura.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>