esp_scsi: make number of tags configurable

Add a field 'num_tags' to the esp structure to allow drivers
to overwrite the number of avialable tags if required.
Default is ESP_DEFAULT_TAGS.

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