uwb: init beacon cache entry before registering uwb device

Make sure the uwb_dev->bce entry is set before calling uwb_dev_add in
uwbd_dev_onair so that usermode will only see the device after it is
properly initialized.  This fixes a kernel panic that can occur if
usermode tries to access the IEs sysfs attribute of a UWB device before
the driver has had a chance to set the beacon cache entry.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>