can: pcan_usb_core: fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning by
moving the "default" to the end of the "switch" statement and explicitly adding
a break statement instead of letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115


Reported-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: http://lore.kernel.org/r/aab7cf16bf43cc7c3e9c9930d2dae850c1d07a3c.1605896059.git.gustavoars@kernel.org


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
[mkl: move default to end]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>