Revert "usb: dwc2: host: Setting qtd to NULL after freeing it"

This reverts commit b0d65902.

The reverted commit does nothing but adding two unnecessary lines
of code.  It sets a local variable to NULL in two functions, but
that variable is not used anywhere in the rest of those functions.
This is just confusing, so let's remove it.

Cc: Vardan Mikayelyan <mvardan@synopsys.com>
Cc: John Youn <johnyoun@synopsys.com>
Cc: Douglas Anderson <dianders@chromiun.org>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>