modem: cmux: optimize `modem_cmux_frame` structure padding

Downsize `dlci_address` field from `uint16_t` to `uint8_t` in
`modem_cmux_frame` for better memory alignment.

Note: The maximum value of `dlci_address` is 63, which fits
within `uint8_t`.

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