sata_sil24: Use const

Reduce data by using const.

$ size drivers/ata/sata_sil24.o*
   text	   data	    bss	    dec	    hex	filename
  12764	    614	   2688	  16066	   3ec2	drivers/ata/sata_sil24.o.new
  12320	   1058	   2688	  16066	   3ec2	drivers/ata/sata_sil24.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>