Fix bug where PPM rx stops working on sparky or CC3D when using motor

PWM rate > 500 (brushed motor mode).

Fix is the same as for OneShot since both brushed motors and oneshot use
an 8mhz timer.

Fixes #58

src/main/drivers/pwm_mapping.h

@@ -35,6 +35,8 @@
 
 #define PWM_TIMER_MHZ 1
 #define ONESHOT125_TIMER_MHZ 8
+#define PWM_BRUSHED_TIMER_MHZ 8
+
 
 typedef struct sonarGPIOConfig_s {
     GPIO_TypeDef *gpio;