staging: dwc2: fix dwc2_hcd_qtd_add()

The logic in dwc2_hcd_qtd_add() was a bit messy, and one of the
error exit paths was broken. Fix it up.

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