pwm: jz4740: Enhance precision in calculation of duty cycle

Calculating the hardware value for the duty from the hardware value of
the period resulted in a precision loss versus calculating it from the
clock rate directly.

(Also remove a cast that doesn't really need to be here)

Fixes: f6b8a570 ("pwm: Add Ingenic JZ4740 support")
Cc: <stable@vger.kernel.org>
Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>