pwm: imx27: Fix overflow for bigger periods

The second parameter of do_div is an u32 and NSEC_PER_SEC * prescale
overflows this for bigger periods. Assuming the usual pwm input clk rate
of 66 MHz this happens starting at requested period > 606060 ns.

Splitting the division into two operations doesn't loose any precision.
It doesn't need to be feared that c / NSEC_PER_SEC doesn't fit into the
unsigned long variable "duty_cycles" because in this case the assignment
above to period_cycles would already have been overflowing as
period >= duty_cycle and then the calculation is moot anyhow.

Fixes: aef1a379 ("pwm: imx27: Fix rounding behavior")
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Tested-by: Johannes Pointner <johannes.pointner@br-automation.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-imx27.c

@@ -235,8 +235,9 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	period_cycles /= prescale;
 	c = clkrate * state->duty_cycle;
-	do_div(c, NSEC_PER_SEC * prescale);
+	do_div(c, NSEC_PER_SEC);
 	duty_cycles = c;
+	duty_cycles /= prescale;
 
 	/*
 	 * according to imx pwm RM, the real period value should be PERIOD