rtlwifi: pci: Fix oops on driver unload

On Fedora systems, unloading rtl8192ce causes an oops. This patch fixes the
problem reported at https://bugzilla.redhat.com/show_bug.cgi?id=852761

.

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