Input: elantech - make arrays debounce_packet static, reduces object code size

Don't populate the arrays debounce_packet on the stack, instead make
them static.  Makes the object code smaller by over 870 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  30553	   9152	      0	  39705	   9b19	drivers/input/mouse/elantech.o

After:
   text	   data	    bss	    dec	    hex	filename
  29521	   9312	      0	  38833	   97b1	drivers/input/mouse/elantech.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>