hdlc_ppp: carrier detect ok, don't turn off negotiation

hdlc_ppp: carrier detect ok, don't turn off negotiation

Sometimes when physical lines have a just good noise to make the protocol
handshaking fail, but the carrier detect still good. Then after remove of
the noise, nobody will trigger this protocol to be start again to cause
the link to never come back. The fix is when the carrier is still on, not
terminate the protocol handshaking.

Signed-off-by: Denis Du <dudenis2000@yahoo.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>