scsi: lpfc: Adjust default value of lpfc_nvmet_mrq

The current default for async hw receive queues is 1, which presents
issues under heavy load as number of queues influence the available
async receive buffer limits.

Raise the default to the either the current hw limit (16) or the number
of hw qs configured (io channel value).

Revise the attribute definition for mrq to better reflect what we do for
hw queues. E.g. 0 means default to optimal (# of cpus), non-zero
specifies a specific limit. Before this change, mrq=0 meant target mode
was disabled. As 0 now has a different meaning, rework the if tests to
use the better nvmet_support check.

Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>