net: ppp: fsm: use size of whole structure instead of each field
net: ppp: fsm: use size of whole structure instead of each field

Just improve readability by converting to sizeof(ppp).

Signed-off-by: Marcin Niestroj <63506c06cfbc47ace147db1702f6e751f5ac2132@grinn-global.com>

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