drivers: led_pwm: fix overflow in set_brightness

The set_brightness function of the led_pwm driver uses a default PWM
period (defined in the pwms DT property) to compute a pulse passed to
the pwm_set_pulse_dt function. If this default period is greater than
2^32/100 nanoseconds (about 43 milliseconds) then the calculation may
overflow.

This patch prevents this overflow by running the pulse computation under
a cast with a larger type (uint64_t).

Reported-by: Scott Worley <scott.worley@microchip.com>
Signed-off-by: Simon Guinot <simon.guinot@seagate.com>