drivers: pwm: nrf_sw: drop broken runtime timer clock scaling

There is some kind of runtime clock scaling implemented when period
cycles exceed 16-bit value. This is broken, because configured PWM has
far higher frequency in that case. Replace that mechanism with simply
veryfying if requested PWM period does not exceed 16-bit value.

Suggested-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>