can: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This patch fixes the following warnings:

drivers/net/can/peak_canfd/peak_pciefd_main.c:668:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/net/can/spi/mcp251x.c:875:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/net/can/usb/peak_usb/pcan_usb.c:422:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/net/can/at91_can.c:895:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/net/can/at91_can.c:953:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/net/can/usb/peak_usb/pcan_usb.c: In function ‘pcan_usb_decode_error’:
drivers/net/can/usb/peak_usb/pcan_usb.c:422:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (n & PCAN_USB_ERROR_BUS_LIGHT) {
      ^
drivers/net/can/usb/peak_usb/pcan_usb.c:428:2: note: here
  case CAN_STATE_ERROR_WARNING:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.

Notice that in some cases spelling mistakes were fixed.
In other cases, the /* fall through */ comment is placed
at the bottom of the case statement, which is what GCC
is expecting to find.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

drivers/net/can/peak_canfd/peak_pciefd_main.c

@@ -660,7 +660,7 @@ static int pciefd_can_probe(struct pciefd_board *pciefd)
 		pciefd_can_writereg(priv, CANFD_CLK_SEL_80MHZ,
 				    PCIEFD_REG_CAN_CLK_SEL);
 
-		/* fallthough */
+		/* fall through */
 	case CANFD_CLK_SEL_80MHZ:
 		priv->ucan.can.clock.freq = 80 * 1000 * 1000;
 		break;