iscsi class: fix get_host_stats return code when not supported

When the get_host_stats call was not supported we were
returing EINVAL. This has us return ENOSYS, because for
software iscsi drivers where there is no host it is ok to not
have this callout.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Acked-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>