ISDN: Fix isdnhdlc for one byte hdlc packets

Normally HDLC packets contain more as one byte (e.g a X25/X75 header).
But if you use plain HDLC framing, the current code do not encode
1 byte payloads, this patch fix that.

Signed-off-by: Karsten Keil <keil@b1-systems.de>