pwm: rcar: Simplify multiplication/shift logic

- Remove the superfluous cast; the multiplication will yield a 64-bit
    result due to the "100ULL" anyway,
  - "a * (1 << b)" == "a << b".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>