drivers: can: loopback: Use thread to send frames.

The loopback driver is a simple driver that can be used to
test CAN subsystems. The actual implementation sends frames
in the same thread that calls the send function.
Some libraries have problems with that behavior.
This PR implements a dedicated thread that calls the callback
for the receiving functions and a msgq in between the sender
and the TX thread.

Signed-off-by: Alexander Wachter <alexander@wachter.cloud>