modem_chat: Allow timeout script chat commands

Modem chat scripts should be able to add a script command which
is simply a delay. The validation of chat scripts currently
disallows chat script commands which only have a timeout set.

Update modem_chat to allow modem chat script commands which
only have a timeout. Additionally do a bit of cleanup to
remove duplicate code.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>