UWB: fix sysfs warning on HWA device unplug.

In the disconnect routine for the hwa_hc interface, it calls
uwb_pal_unregister to unregister itself from the UWB subsystem.  This
function attempts to clean up the link to the host controller directory in
the device's UWB radio control interface directory.  If the disconnect
routine for the radio control interface has already run, the uwb directory
will be gone so the call to sysfs_remove_link generates a warning.

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