clocksource: sh_tmu: Fix channel IRQ retrieval in legacy case

In the legacy platform data case each TMU platform device handles a
single channel with a single IRQ for the platform device. Retrieve the
IRQ using the logical channel number instead of the hardware channel
number.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>