Bug with PWM when frequency is reduced

[mstrens]

For STM32, the timer used by PWM is a 16 bit.
In file spindle_control.c a new type (SPINDLE_PWM_TYPE) has been define to take care of this.

Still in file stepper.c, there are 2 fields that are used to store rpm values (converted in PWM timer values). Those 2 fields should be defined as SPINDLE_PWM_TYPE too.
Otherwise, when frequency is reduced to e.g. 2000Hz, the max value for the timer is 500 and exceed a uint8_t and stepper.c generates wrong PWM.

The 2 fields to redefine are :
Here my changes:

#ifdef VARIABLE_SPINDLE
//uint8_t spindle_pwm; replaced by mstrens for STM32 by next line
SPINDLE_PWM_TYPE spindle_pwm; // SPINDLE_PWM_TYPE is Uint16_t for STM32 and uint8_t for AVR
#endif

and the second:

#ifdef VARIABLE_SPINDLE
float inv_rate; // Used by PWM laser mode to speed up segment calculations.
//uint8_t current_spindle_pwm; // replaced by mstrens by next line for STM32
SPINDLE_PWM_TYPE current_spindle_pwm; // for STM32 SPINDLE_PWM_TYPE must be uint16_t
#endif

[robomechs]

fixed