mISDN: Using FLG_ACTIVE flag to determine if layer 1 is active or not.

We already have the flag for L1 active, so we should use it.
L2 will be solved in a later patch.

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>