modem: cmux: make work buffer size configurable

A Kconfig item is added and the buffer
is embedded into the modem_cmux struct.
The default value is increased from 16 to 64 bytes in
an effort to reduce the number of modem_pipe_receive() calls.

CONFIG_MODEM_CHAT_LOG_BUFFER is renamed
to CONFIG_MODEM_CHAT_LOG_BUFFER_SIZE for consistency.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>