modem: modem_cellular: Configurable MTU for CMUX

Allow configuring MTU for CMUX.
Some AT manual and specification define this as a
frame size. Linux ldattach default to 127 bytes,
3GPP TS 27.010 defaults to 31.

We should limit our CMUX frames to a size that
remote end is capable of handling.
Linux silently drops oversized frames.

Also, remove MODEM_CELLULAR_CMUX_MAX_FRAME_SIZE as
this was only limiting a buffer sizes, and resulted
CMUX frames to be capped to same value.
Use MODEM_CMUX_WORK_BUFFER_SIZE and MODEM_CMUX_MTU instead.

Also rename CONFIG_MODEM_CELLULAR_CHAT_BUFFER_SIZES to
CONFIG_MODEM_CELLULAR_CHAT_BUFFER_SIZE as it is now
only used as a Chat module. DLCI pipes use
CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE.

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