fpga: mgr: altera-ps-spi: make array dummy static, shrinks object size

Don't populate the const array dummy on the stack but instead
make it static. Makes the object code smaller by 26 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   7371	   2032	      0	   9403	   24bb	drivers/fpga/altera-ps-spi.o

After:
   text	   data	    bss	    dec	    hex	filename
   7281	   2096	      0	   9377	   24a1	drivers/fpga/altera-ps-spi.o

(gcc version 8.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Alan Tull <atull@kernel.org>
Acked-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>