modem: modem_cmux: Make async connect/disconnect stateful

This commit adds a check to the async connect and disconnect
functions to validate the CMUX is not already connected
or disconnected respectively. This was already part of the
sync connect and disconnect functions, so the sync functions
now simply wrap the async functions to avoid duplicate code.

Signed-off-by: Bjarki Arge Andreasen <bjarkix123@gmail.com>