s390/qeth: remove QAOB's pointer to its TX buffer

Maintaining a pointer inside the aob's user-definable area is fragile
and unnecessary. At this stage we only need it to overload the buffer's
state field, and to access the buffer's TX queue.

The first part is easily solved by tracking the aob's state within the
aob itself. This also feels much cleaner and self-contained.
For enabling the access to the associated TX queue, we can store the
queue's index in the aob.

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