Describe the bug
When assert is activated there may be assert failed when using Tone or Servo.
assert_param(IS_TIM_CC1_INSTANCE(htim->Instance));
This is due to the fact that when using timer instances without output (like TIM6 and TIM7 specially used for Tone and Servo) in TIMER_OUTPUT_COMPARE,setMode()API required a channel even if it is not used.
This was made like this to simplify the HardwareTimer driver, and there is no functional issue,
but as there is an assert failed reported when assert is activated, this should be fixed.

To Reproduce
Enable switchUSE_FULL_ASSERT
Compile 'knob' servo example.