Fix actuator period to allow for lower output rates than 16Hz

Frequencies lower than 16Hz were not possible because of integer overflow. This resolves that problem by scaling the PWM resolution frequency. Division by 2 (i.e. a right bitshift) is used to scale the values, in order to keep PIOS_Servo_Set performance costs relatively unchanged.

Tested with logic analyzer on F1, F3, and F4. Works at frequencies above and below 16Hz.