drivers: modem: modem_cellular: extend CMUX timeout

This commit adds a timeout of 300ms to the generic (gsm_ppp) init
chat script. This delay is required for some modems (discovered on
a Telit ME910G1-WW) to allow it to enter CMUX mode. Without this
delay, the modem simply refuses to respond to any CMUX commands.

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