rtc: rtc-sh: Correct sh_rtc_set_time() for some SH-3 parts.

Some SH-3 parts (SH7720 and SH7705 at least) need to have the
start bit explicitly cleared, as the reset is not enough. This
is safe across all parts, so simply clear the start bit in
the sh_rtc_set_time() path.

Signed-off-by: Markus Brunner <super.firetwister@gmail.com>
Signed-off by: Mark Jonas <toertel@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>