Bluetooth: Use single_open() for inquiry cache within debugfs

The inquiry cache information in debugfs should be using seq_file support
and not allocating memory on the stack for the string. Since the usage of
these information is really seldom, using single_open() for it is good
enough.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>