Consider API change to deal with compiler warnings around array slices

[humphrey-xmos]

lib_device_control uses function parameters such as client interface control i_control[], which generates warnings...

​warning: argument 2 of `i2c_client' slices interface preventing analysis of its parallel usage [-Wunusual-code]

I take it these warnings are not very significant as much of the code uses pragmas to silence them, is that correct?

Consider

• adding an optional config header with a variable such as NUM_CONTROL_INTERFACES
• and updating the API to client interface control i_control[NUM_CONTROL_INTERFACES]
• removing the pragmas

[humphrey-xmos]

This also becomes an issue if combinable or distributable is enabled on the control endpoint. It will not compile if either mode is added to the control endpoint server (with sliced arrays)

[ed-xmos]

I have successfully done this in #134.
It allows the control tasks to be distributed (eg. I2C + i2c_control_client) and no longer needs the warnings to be disabled.

It requires NUM_CONTROL_INTERFACES to be set. This does mean only one instance of lib_device_control may be instantiated in the project, but this feels OK.