sh: Fix sh7264 clock speed and related serial setting

The generalised calculation of the serial bit rate reg also applies
to sh7264, it was just the clock speed that was set incorrectly.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>