HID: prodikeys: make array keys static const, makes object smaller

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>