Don't populate the array keys on the stack but instead make it
static const. Makes the object code smaller by 166 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  18931	   5872	    480	  25283	   62c3	drivers/hid/hid-prodikeys.o

After:
   text	   data	    bss	    dec	    hex	filename
  18669	   5968	    480	  25117	   621d	drivers/hid/hid-prodikeys.o

(gcc version 9.2.1, amd64)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>