Next step of my ->init removal series.  sg does a few to much wierd
things with it's global array thay I prefer to leave it to Doug to
get rid of it (if he wants to), but this patch at least gets rid of
sg_init.

Move the register_chrdev to init_sg - open properly checks whether
the device exists so this doesn't cause any harm.  Remove the initial
allocation of the device array - the resizing code in sg_attach will
properly take care of it when called the first time.

Tested with a DVD reader/CD writer combination and ide-scsi.