af_iucv: change net_device handling for HS transport

This patch saves the net_device in the iucv_sock structure during
bind in order to fasten skb sending.
In addition some other small improvements are made for HS transport:
   - error checking when sending skbs
   - locking changes in afiucv_hs_callback_txnotify
   - skb freeing in afiucv_hs_callback_txnotify
And finally it contains code cleanup to get rid of iucv_skb_queue_purge.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>