Remove tpg/enable attribute.  Add fabric ops enable_tpg implementation
instead.

Link: https://lore.kernel.org/r/20210910084133.17956-5-d.bogdanov@yadro.comReviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>