esp_scsi: enable CONFIG2_FENAB for am53c974

CONFIG2_FENAB ('feature enable') changed definition between chip
revisions, from 'Latch SCSI Phase' to 'Latch SCSI Phase, display
chip ID upon reset, and enable 24 bit addresses'.
So only enable it for am53c974 where we know what it's doing.

Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>