Don't populate the const read-only arrays spi_test_unit_ready and
spi_test_unit_ready on the stack but instead make them static. Makes the
object code smaller by over 100 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  40171	  12832	    128	  53131	   cf8b	drivers/scsi/scsi_transport_spi.o

After:
   text	   data	    bss	    dec	    hex	filename
  39922	  12976	    128	  53026	   cf22	drivers/scsi/scsi_transport_spi.o

(gcc version 7.2.0 x86_64)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>