staging: rtl8712: Fix oops on dongle removal if firmware is not available

When firmware is not available, net device is not created. Upon
disconnect, we must check for net device existence before trying to
release net device private data.

Signed-off-by: Thomas Vegas <thomas@grouk.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>