CN - drv_timer.c changes.
Reduces the time spent in the interrupt handler.
Some variable types also changed from uint8_t to int or unsigned int to help improve code size/speed.
Rudimentary measurement of time spent in the handler seems to indicate a reduction from ~8us down to ~5us (excluding time spent in callbacks).
CN - Change enum names to suit Baseflight coding style (lowercase -> uppercase).
Merge pull request #146 from ChrisNisbet01/drv_timer

CN - drv_timer.c changes.