It should fix the OOPS on attach.

This fixes a silly error where I fail to initialize a pointer early enough
for the scanning code.  If this isn't a perfect example of why
scsi_register() and scsi_add_host() aren't two separate functions, I don't
know what is.  :)

Oh, and I added a couple of comments, too.

  -  Fix an OOPS by moving the setting of the hostdata[] pointer to _before_
     the device scan starts.