staging: dwc2: fix potential use after free

dwc2_process_non_isoc_desc() can potentially free the qtd, so null
out the qtd pointer if the call fails so we don't try to access it
later

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>