net/lapb: fix t1 timer handling for LAPB_STATE_0

1. DTE interface changes immediately to LAPB_STATE_1 and start sending
   SABM(E).

2. DCE interface sends N2-times DM and changes to LAPB_STATE_1
   afterwards if there is no response in the meantime.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>