USB: HWA: fix device probe failure

This patch fixes a race condition that caused the HWA_HC interface probe
function to occasionally fail.  The HWA_HC would attempt to register
itself with the HWA_RC by searching for a uwb_rc class device with the
same parent device ptr.  If the probe function for the HWA_RC interface
had yet to run, the uwb_rc class device would not have been created
causing the look up to fail and the HWA_HC probe function to return an
error causing the device to be unusable.

The fix is for the HWA to delay registering with the HWA_RC until
receiving the command from userspace to start the wireless channel.  It
is the responsibility of userspace to ensure that the uwb_rc class
device has been created before starting the HWA channel.

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