libata: make static arrays const, reduces object code size

Don't populate const arrayis on the stack, instead make them static.
Makes the object code smaller by over 260 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  64864	   5948	   4128	  74940	  124bc	drivers/ata/libata-scsi.o

After:
   text	   data	    bss	    dec	    hex	filename
  64183	   6364	   4128	  74675	  123b3	drivers/ata/libata-scsi.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>