[Grbl_Esp32 PR#338] Simple speedup for spindle PWM

This is the opening salvo in an effort to improve the performance of PWM by optimizing the driver code for the ESP32’s LED PWM hardware. In this first checkin, we simply avoid the need to call ledcRead() by saving the old duty cycle value in memory, thus avoiding both a function call and an I/O read operation – as I/O reads cost lots of CPU cycles compared to memory reads.