nvme: fixup comment for nvme RDMA Provider Type

PRTYPE is the provider type, not the QP service type.

Fixes: eb793e2c

 ("nvme.h: add NVMe over Fabrics definitions")
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>