rtlwifi: rtl8192cu: Fix NULL dereference BUG when using new_id

When the new_id entry in /sysfs is used for a foreign USB device, rtlwifi
BUGS with a NULL pointer dereference because the per-driver configuration
data is not available. The probe function has been restructured as
suggested by Ben Hutchings <bhutchings@solarflare.com>.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>