Bluetooth: make hw_err static, reduces object code size

Don't populate the const array hw_err on the stack but instead make
it static. Makes the object code smaller by 45 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
 100880	  21090	   1088	 123058	  1e0b2	linux/net/bluetooth/hci_core.o

After:
   text	   data	    bss	    dec	    hex	filename
 100739	  21186	   1088	 123013	  1e085	linux/net/bluetooth/hci_core.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>