esp_scsi: correctly detect am53c974

The am53c974 returns the same ID as the FAS236, but implements
things slightly differently. So detect the am53c974 by checking
for ESP_CONFIG4 register.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>