scsi: ibmvfc: Define generic queue structure for CRQs

The primary and async CRQs are nearly identical outside of the format and
length of each message entry in the dma mapped page that represents the
queue data. These queues can be represented with a generic queue structure
that uses a union to differentiate between message format of the mapped
page.

This structure will further be leveraged in a followup patcheset that
introduces Sub-CRQs.

Link: https://lore.kernel.org/r/20210106201835.1053593-2-tyreld@linux.ibm.com
Reviewed-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>