samd21: port_disable_tick() should disable event channel

[dhalbert]

Fixes #7105.

On SAMD21, port_disable_tick() did not finish the job of disabling the _tick_event_channel.

Also added a check to avoid doing anything if _tick_event_channel was already disabled.

Before this change, editing a file and causing an autoreload would use up a bunch of event channels:
(this is from some temporary logging I added:

[code.py edited here]
Code stopped by auto-reload. Reloading soon.
>port_disable_tick, _tick_event_channel: 0
<port_enable_tick, _tick_event_channel: 0
>port_disable_tick, _tick_event_channel: 0
<port_enable_tick, _tick_event_channel: 1
>port_disable_tick, _tick_event_channel: 1
<port_enable_tick, _tick_event_channel: 2
>port_disable_tick, _tick_event_channel: 2
<port_enable_tick, _tick_event_channel: 3
>port_disable_tick, _tick_event_channel: 3
<port_enable_tick, _tick_event_channel: 4
>port_disable_tick, _tick_event_channel: 4
<port_enable_tick, _tick_event_channel: 5
>port_disable_tick, _tick_event_channel: 5
<port_enable_tick, _tick_event_channel: 6
>port_disable_tick, _tick_event_channel: 6

Now it reuses the same channel:

[code.py edited here]
Code stopped by auto-reload. Reloading soon.
>port_disable_tick, _tick_event_channel: 0
<port_enable_tick, _tick_event_channel: 0
>port_disable_tick, _tick_event_channel: 0
<port_enable_tick, _tick_event_channel: 0
>port_disable_tick, _tick_event_channel: 0
<port_enable_tick, _tick_event_channel: 0

[tannewt]

Thank you!