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>

include/zephyr/modem/cmux.h

@@ -111,7 +111,7 @@ struct modem_cmux_dlci {
 };
 
 struct modem_cmux_frame {
-	uint16_t dlci_address;
+	uint8_t dlci_address;
 	bool cr;
 	bool pf;
 	uint8_t type;