pwm: atmel: Replace loop in prescale calculation by ad-hoc calculation

The calculated values are the same with the modified algorithm. The only
difference is that the calculation is a bit more efficient.

Acked-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>