sh: timer: Mask bit of timer prescaler

timer_init function sets timer prescaler bit.
The previous code so did not mask this bit, this function was to overwrite
the bit. This will fix this problem.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>