rtl8187: Fix for kernel oops when unloading with LEDs enabled

When rtl8187 is unloaded and CONFIG_RTL8187_LEDS is set, the kernel
may oops when the module is unloaded as the workqueue for led_on was
not being cancelled.

This patch fixes the problem reported in
http://marc.info/?l=linux-wireless&m=124742957615781&w=2.

Reported-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger>
Signed-off-by: John W. Linville <linville@tuxdriver.com>