Bluetooth: hci_qca: fix debugfs registration

Since commit 3e4be65e ("Bluetooth: hci_qca: Add poweroff support
during hci down for wcn3990"), the setup callback which registers the
debugfs interface can be called multiple times.

This specifically leads to the following error when powering on the
controller:

	debugfs: Directory 'ibs' with parent 'hci0' already present!

Add a driver flag to avoid trying to register the debugfs interface more
than once.

Fixes: 3e4be65e

 ("Bluetooth: hci_qca: Add poweroff support during hci down for wcn3990")
Cc: stable@vger.kernel.org	# 4.20
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>