modem: cmux: Define encoding and decoding functions for commands

Instead of relying non-standard compiler behavior, define
encode and decode functions for all CMUX command structures.

Final command is encoded into a shared buffer, because it is
always copied directly to TX ringbuffer.

Added also functions to validate commands.

Signed-off-by: Seppo Takalo <[email protected]>

Author: SeppoTakalo