[SCSI] esp: fix instance numbering.

Because the ->unique_id is set too late, the ESP scsi host
instance numbers in the kernel log during probing are
wrong.

Bug reported by Meelis Roos.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>