pwm: renesas-tpu: Improve precision of period and duty_cycle calculation

Dividing by the result of a division looses precision. Consider for example
clk_rate = 33000000 and period_ns = 500001. Then

	clk_rate / (NSEC_PER_SEC / period_ns)

has the exact value 16500.033, but in C this evaluates to 16508. It gets
worse for even bigger values of period_ns, so with period_ns = 500000001,
the exact result is 16500000.033 while in C we get 33000000.

For that reason use

	clk_rate * period_ns / NSEC_PER_SEC

instead which doesn't suffer from this problem. To ensure this doesn't
overflow add a safeguard check for clk_rate.

Note that duty > period can never happen, so the respective check can be
dropped.

Incidentally this fixes a division by zero if period_ns > NSEC_PER_SEC.
Another side effect is that values bigger than INT_MAX for period and
duty_cyle are not wrongly discarded any more.

Fixes: 99b82abb ("pwm: Add Renesas TPU PWM driver")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>