Bluetooth: Expose auto_accept_delay debugfs only when SSP is supported

The auto_accept_delay debugfs entry is only valid for BR/EDR capable
controllers that also support SSP. If SSP is not available or it is
a LE-only single mode controller this value has no affect and so do
not expose it.

Since the value can be actually changed, switch the permissions
to 0644 to clearly indicate that the value is indeed writeable.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>