kinetis: fix PPM decoding

On kinetis, the TPM_STATUS_TOF was used to detect missed interrupts for
PPM decoding. However this was not correct, because the TOF bit was set on
each timer overflow. This happened regularly after every 64ms interval,
which meant that most PPM frames were just discarded.

I have not found any equivalent register/solution that is used on STMF4,
so this just removes the TOF handling. However there is now no way to
detect missed edges/interrupts!

src/drivers/kinetis/drv_hrt.c

@@ -346,11 +346,6 @@ static void hrt_ppm_decode(uint32_t status)
 	uint16_t interval;
 	unsigned i;
 
-	/* if we missed an edge, we have to give up */
-	if (status & TPM_STATUS_TOF) {
-		goto error;
-	}
-
 	/* how long since the last edge? - this handles counter wrapping implicitly. */
 	width = count - ppm.last_edge;
 
@@ -510,7 +505,7 @@ hrt_tim_isr(int irq, void *context, void *arg)
 #ifdef HRT_PPM_CHANNEL
 
 	/* was this a PPM edge? */
-	if (status & (STATUS_PPM | TPM_STATUS_TOF)) {
+	if (status & (STATUS_PPM)) {
 		hrt_ppm_decode(status);
 	}