AP_IOMCU: allow up to 16 channels of servo data to be sent to the iomcu

[andyp1per]

Fixes https://github.com/ArduPilot/ardupilot/issues/26851

[andyp1per]

4.5.0beta3 using dshot without the whole SBUS change:

With the SBUS change

So losing 128 bytes of memory and 48 bytes of stack

[rmackay9]

yay! Thanks so much for this!

[tridge]

@rmackay9 will test

[tridge]

we need to work out where the duplicates on chan13 came from, maybe gdb on IOMCU looking in SBUS out function to see where it got the value from. Concern is this is a crash bug

[rmackay9]

I've confirmed that this fixes the most recent issue we've seen with the SBUS output.

So to re-iterate the testing procedure, I setup two CubeOrangePluses where the 1st autopilot's SBUS output was connected to the 2nd autopilot's RCIN.

Below is a screen shot of the 1st autopilot's servo output and we can see that it is copying RCIN 6 to four output channels.

Then on the 2nd autopilot we can see that the correct PWMs are appearing as SBUS input.

[rmackay9]

@andyp1per,

After this change there is just a single use of IOMCU_MAX_CHANNELS on line 1152 of iofirmware.cpp. I'm just guessing but should that line also be changed?

[andyp1per]

After this change there is just a single use of IOMCU_MAX_CHANNELS on line 1152 of iofirmware.cpp. I'm just guessing but should that line also be changed?

No, that one is correct