scsi: mpt3sas: Introduce struct mpt3sas_nvme_cmd

Make _base_build_nvme_prp() easier to read by introducing a structure
to access NVMe command fields.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Sathya Prakash <sathya.prakash@broadcom.com>
Cc: Chaitra P B <chaitra.basappa@broadcom.com>
Cc: Suganath Prabu Subramani <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>