serial: sh-sci: Change checking for error rate of HSCIF

This changes negative values of error rate to be checked, because these
values are valid as error rate. And this changes in the process of adopting
a value close to 0.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>