s390/qeth: remove various redundant code

1. tracing iob->rc makes no sense when it hasn't been modified by the
   callback,
2. the qeth_dbf_list is declared with LIST_HEAD, which also initializes
   the list,
3. the ccwgroup core only calls the thaw/restore callbacks if the gdev
   is online, so we don't have to check for it again.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>