USB: serial: iuu_phoenix: drop unused URB submission results

The driver is submitting URBs in various completion callbacks without
bothering to log errors yet still assigned the return value to temporary
variables. Let's drop those temporaries.

This suppresses the corresponding W=1 (-Wunused-but-set-variable)
warnings.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>