modem: chat: Optimize const usage

This commit removes const declarations for members
inside structures used for chat scripts to allow for
modifying them at runtime.

Macros like MODEM_CHAT_SCRIPT_DEFINE() cover the
whole struct when declaring them const, ensuring
they remain placed in ROM.

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