staging: rtl8192u: r8192U_core: Fix driver_info dereference as a null pointer

Fix possible use of use of driver_info as a null pointer in
query_rxdesc_status()
This could happen if stats->RxIs40MHzPacket still has the
default value of zero.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>