HID: elan: Make array buf static, shrinks object size

Don't populate the array buf on the stack but instead make it
static. Makes the object code smaller by 43 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   7769	   1520	      0	   9289	   2449	drivers/hid/hid-elan.o

After:
   text	   data	    bss	    dec	    hex	filename
   7662	   1584	      0	   9246	   241e	drivers/hid/hid-elan.o

(gcc version 8.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>