s390/qdio: remove a forward declaration

Shift the definition of tiqdio_airq around, so that it doesn't require a
forward declaration for tiqdio_thinint_handler().

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