nvmet-fc: fix endianess annoations for nvmet_fc_format_rsp_hdr

The passed in desc_len is a big endian value, so mark it as such.

Found by sparse.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>