can: gs_usb: gs_can_start_xmit(): zero-initialize hf->{flags,reserved}

No information is deliberately sent in hf->flags in host -> device
communications, but the open-source candleLight firmware echoes it
back, which can result in the GS_CAN_FLAG_OVERFLOW flag being set and
generating spurious ERRORFRAMEs.

While there also initialize the reserved member with 0.

Fixes: d08e973a ("can: gs_usb: Added support for the GS_USB CAN devices")
Link: https://lore.kernel.org/all/20220106002952.25883-1-brian.silverman@bluerivertech.com
Link: https://github.com/candle-usb/candleLight_fw/issues/87


Cc: stable@vger.kernel.org
Signed-off-by: Brian Silverman <brian.silverman@bluerivertech.com>
[mkl: initialize the reserved member, too]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>