s390/ap: make tapq gr2 response a struct

This patch introduces a new struct ap_tapq_gr2 which covers
the response in GR2 on TAPQ invocation. This makes it much
easier and less error-prone for the calling functions to
access the right field without shifting and masking.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>