can: peak/pcie_fd: fix potential bug in restarting tx queue

Don't rely on can_get_echo_skb() return value to wake the network tx
queue up: can_get_echo_skb() returns 0 if the echo array slot was not
occupied, but also when the DLC of the released echo frame was 0.

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>