modem: modem_cellular: Adjust chat scripts

This commit adjust the chat scripts for the simcom
sim7080 and gsm_ppp compatibles to fix an issue found
when trying to use a sim7080 modem. The CMUX command
includes optional parameters which are not identical
for all modems, so the AT+CMUX command has been adjusted
to only contain the mandatory parameters.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>