net: l2: ppp: fix packet length for sending Term-Req and Term-Ack
net: l2: ppp: fix packet length for sending Term-Req and Term-Ack

Each PPP packet sent on wire needs to have at least 4 bytes length. Set
that length for outgoing Term-Req and Term-Ack packets. Also update
length validation to check for at least 4 bytes instead of at least 1
byte.

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

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