modem: cmux: Define macros for header size

Define CMUX_HEADER_SIZE macro that is 6 or 7 bytes depending on
MTU.

Remove the previous CMUX_FRAME_SIZE_MAX that was a bit misleading
as it meant only the header size without data and was fixed to 7 bytes.

Redefine new macros CMUX_FRAME_SIZE_MAX and CMUX_FRAME_SIZE_MIN which
are actually frame sizes with data included.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>