drivers: console: gsm_mux: fix length indicator
drivers: console: gsm_mux: fix length indicator

The MSB and LSB were inverted for length > 127
Fixes #41077

Signed-off-by: Guillaume Lager <e71b22248a48be36ec5c3996607e6ee5cb1844f3@innoseis.com>

galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr