Fix MMU tool change

[3d-gussner]

Fix Tool change be4104a

At some point we broke the tool change. It shows always ?><new tool number> even when the tool is known.
The tool has been reset all the time even during a tool change while unloading and not ONLY when a final UNLOAD has been requested.

Starting with F1 loaded:

Then change from T0 to T1

Before:

After:

Tested on MK404

Test steps to the second commit:

1. Flash FW.3.14.0

2. Connect with serial terminal

3. Enable MMU

4. After power up the printer doesn't know which filament is loaded and shows F?

5. Preheat the nozzle only Settings -> Temperature -> Nozzle

6. Send T0 to load the filament from slot 1 on the MMU

7. Now you can see on LCD F1

8. Send T1 to load the 2nd filament

9. This will unload the first filament and then load the second

10. During this operation watch the LCD screen
    12.1. It witches from F1 to ?>2 and finally shows F2 when the T1 command has finished

11. The ? indicates that the filament slot is UNKNOWN
    13.1. This is correctly shown when the printer starts up
    13.2. BUT it should show during the filament change {current known filament}>{new selected filament}.
    This has been working with FW 3.12.2 and MMU 1.0.6 and got broken with FW3.13.x and MMU 3.x.x

12. Flash firmware with this PR

13. repeat steps 2 to 12

14. Now you see it switching from F1 to 1>2 and finally shows F2 when the T1 command has finished
    16.1 This is the desired output.

Reasons why that happens is that the current firmware "always" reset the filament to UNKNOWN when unload is executed.
The UNKNOWN state should ONLY be set when the filament actually is unknown, which means unloaded for real and not only if the unload of the filament is temporary to be able to load next filament.

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All values in bytes. Δ Delta to base

Target	ΔFlash	ΔSRAM	Used Flash	Used SRAM	Free Flash	Free SRAM
MK3S_MULTILANG	0	0	248044	5654	5908	2538
MK3_MULTILANG	0	0	247420	5663	6532	2529