Bluetooth: btrtl: Make array extension_sig static, shrinks object size

Don't populate the array extension_sig on the stack but instead make it
static. Makes the object code smaller by 75 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  14325	   4920	      0	  19245	   4b2d	drivers/bluetooth/btrtl.o

After:
   text	   data	    bss	    dec	    hex	filename
  14186	   4984	      0	  19170	   4ae2	drivers/bluetooth/btrtl.o

(gcc version 8.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>