HID: asus: make array 'buf' static const

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

Before:
   text	   data	    bss	    dec	    hex	filename
  14378	   2384	     64	  16826	   41ba	linux/drivers/hid/hid-asus.o

After:
   text	   data	    bss	    dec	    hex	filename
  14296	   2440	     64	  16800	   41a0	linux/drivers/hid/hid-asus.o

(gcc version 7.2.0 x86_64)

[jkosina@suse.cz: change commit message slightly]
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>