set min_elapsed_time based on frequency

Author: Richard Unger

src/sensors/MagneticSensorPWM.cpp

@@ -43,6 +43,8 @@ MagneticSensorPWM::MagneticSensorPWM(uint8_t _pinPWM, int freqHz, int _total_pwm
     // define if the sensor uses interrupts
     is_interrupt_based = false;
 
+    min_elapsed_time = 1000000/freqHz; // set the minimum time between two readings
+
     // define as not set
     last_call_us = _micros();
 }