mISDN: Support DR6 indication in mISDNipac driver

According to figure 39 in PEB3086 data sheet, version 1.4 this indication
replaces DR when layer 1 transition source state is F6.

This fixes mISDN layer 1 getting stuck in F6 state in TE mode on
Dialogic Diva 2.02 card (and possibly others) when NT deactivates it.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Acked-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>