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 <m.niestroj@grinn-global.com>