modem: cmux: apply `static const` to reduce RAM usage

Added `static const` to various `struct modem_pipe_api` and
`frame` in `modem_cmux_connect_handler` to make these structures
immutable and reduce RAM usage.

Corrected a typo, changing "consequtive" to "consecutive".

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>