s390/qeth: don't call INIT_LIST_HEAD() on iob's list entry

INIT_LIST_HEAD() only needs to be called on actual list heads.
While at it clarify the naming of the field.

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