Opta Controller: Get pwm functions used wrong index

[maidnl]

This PR corrects the behavior of Get PWM functions:

uint32_t getPwmPeriod(uint8_t ch);
uint32_t getPwmPulse(uint8_t ch);
float getPwmFreqHz(uint8_t ch);
float getPwmPulsePerc(uint8_t ch);

Those functions accepted as ch parameter a number from 0 to 3. This was not aligned with the setPwm() function that required its ch parameter to be from OA_PWM_CH_0 to OA_PWM_CH_3.
Now the "get" functions accept as channel OA_PWM_CH_0 ... OA_PWM_CH_3 as parameter.
It has been maintained the old behavior (in order to maintain compatibility) so a user can continue to use 0..3 as parameter however this practice is deprecated.