Bluetooth: Fix format arguments warning

Newer GCC versions are a little bit picky about how to deal with format
arguments:

net/bluetooth/hci_sysfs.c: In function ‘hci_register_sysfs’:
net/bluetooth/hci_sysfs.c:418: warning: format not a string literal and no format arguments

It is simple enough to fix and makes the compiler happy.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>