can: mscan: mark expected switch fall-through

can: mscan: mark expected switch fall-through

As -Wimplicit-fallthrough is now enabled the mscan driver shows this warning,
which is fixed by this patch:

drivers/net/can/mscan/mscan.c: In function ‘mscan_start_xmit’:
drivers/net/can/mscan/mscan.c:211:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
  211 |   netif_stop_queue(dev);
      |   ^~~~~~~~~~~~~~~~~~~~~
drivers/net/can/mscan/mscan.c:212:2: note: here
  212 |  case 2:
      |  ^~~~

Link: https://lore.kernel.org/r/20200915223527.1417033-17-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>