staging: dwc2: fix some functions to return a proper error code

Fix some functions called by dwc2_hcd_qtd_add() to return either
a proper error code or 0, instead of somewhat random values.
Then change the caller of dwc2_hcd_qtd_add() to just check the
return value for 0.

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