can: peak_canfd: pucan_handle_can_rx(): fix echo management when loopback is on

Echo management is driven by PUCAN_MSG_LOOPED_BACK bit, while loopback
frames are identified with PUCAN_MSG_SELF_RECEIVE bit. Those bits are set
for each outgoing frame written to the IP core so that a copy of each one
will be placed into the rx path. Thus,

- when PUCAN_MSG_LOOPED_BACK is set then the rx frame is an echo of a
  previously sent frame,
- when PUCAN_MSG_LOOPED_BACK+PUCAN_MSG_SELF_RECEIVE are set, then the rx
  frame is an echo AND a loopback frame. Therefore, this frame must be
  put into the socket rx path too.

This patch fixes how CAN frames are handled when these are sent while the
can interface is configured in "loopback on" mode.

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Link: https://lore.kernel.org/r/20201013153947.28012-1-s.grosjean@peak-system.com
Fixes: 8ac8321e

 ("can: peak: add support for PEAK PCAN-PCIe FD CAN-FD boards")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>