scsi: ibmvfc: Define hcall wrapper for registering a Sub-CRQ

Sub-CRQs are registred with firmware via a hypercall. Abstract that
interface into a simpler helper function.

Link: https://lore.kernel.org/r/20210114203148.246656-6-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>