s390/qdio: remove q->first_to_kick

q->first_to_kick is obsolete, and can be replaced by q->first_to_check.

Both cursors start off at 0. Out of the three code paths that update
first_to_check, the qdio_inspect_queue() path is irrelevant as it
doesn't even touch first_to_kick anymore.
This leaves us with the two tasklet-driven code paths. Here any update
to first_to_check is followed by a call to qdio_kick_handler(), which
advances first_to_kick by the same amount.

So the two cursors will differ only for a tiny moment. Drivers have no
way of deterministically observing this difference, and thus it doesn't
matter which of the cursors we use for reporting an error to q->handler.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>