[INAV 8.0.0] Mixer speed value overflow

[davidk-ro]

In the Mixer tab, setting speed larger than 127us/s causes overflow (e.g. set 128us/s will cause it to display -127us/s)
The value should be an unsigned char 8bit or at least some check so it can't overflow.

[sensei-hacker]

Fixed in #2326 , assuming that gets merged.