Propgate any errors in setting the rate up.

Since the Onshot<->PWM mode is changed based on rate. The
 this signale the higher layers that the operation is invalid

 test case:
   fmu task mode_pwm2cap2
   pwm oneshot -m 0xf -g 0 -d /dev/pwm_output0

   should fail, because all the channels in the group are
   not in the same mode.

src/drivers/stm32/drv_pwm_servo.c

@@ -133,8 +133,7 @@ int up_pwm_servo_set_rate_group_update(unsigned group, unsigned rate)
 		}
 	}
 
-	io_timer_set_rate(group, rate);
-	return OK;
+	return io_timer_set_rate(group, rate);
 }
 
 void up_pwm_update(void)