nvmet: remove op_flags for passthru commands

For passthru commands setting op_flags has no meaning. Remove the code
that sets the op flags in nvmet_passthru_map_sg().

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>