can: peak_pciefd: pucan_handle_status(): fix a potential starvation issue in TX path

can: peak_pciefd: pucan_handle_status(): fix a potential starvation issue in TX path

Rather than just indicating that transmission can start, this patch
requires the explicit flushing of the network TX queue when the driver
is informed by the device that it can transmit, next to its
configuration.

In this way, if frames have already been written by the application,
they will actually be transmitted.

Fixes: 5b42e771c565 ("can: peak/pcie_fd: remove useless code when interface starts")
Link: https://lore.kernel.org/r/20210623142600.149904-1-s.grosjean@peak-system.com
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>