usb: wusbcore: fix panic in wusbhc_chid_set

If no valid CHID value has previously been set on an HWA, writing a
value of all zeros will cause a kernel panic in uwb_radio_stop because
wusbhc->uwb_rc has not been set.  This patch skips the call to
uwb_radio_stop if wusbhc->uwb_rc has not been initialized.

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