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/at91_can.c

@@ -898,7 +898,8 @@ static void at91_irq_err_state(struct net_device *dev,
 				CAN_ERR_CRTL_TX_WARNING :
 				CAN_ERR_CRTL_RX_WARNING;
 		}
-	case CAN_STATE_ERROR_WARNING:	/* fallthrough */
+		/* fall through */
+	case CAN_STATE_ERROR_WARNING:
 		/*
 		 * from: ERROR_ACTIVE, ERROR_WARNING
 		 * to  : ERROR_PASSIVE, BUS_OFF
@@ -947,7 +948,8 @@ static void at91_irq_err_state(struct net_device *dev,
 		netdev_dbg(dev, "Error Active\n");
 		cf->can_id |= CAN_ERR_PROT;
 		cf->data[2] = CAN_ERR_PROT_ACTIVE;
-	case CAN_STATE_ERROR_WARNING:	/* fallthrough */
+		/* fall through */
+	case CAN_STATE_ERROR_WARNING:
 		reg_idr = AT91_IRQ_ERRA | AT91_IRQ_WARN | AT91_IRQ_BOFF;
 		reg_ier = AT91_IRQ_ERRP;
 		break;

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;

drivers/net/can/spi/mcp251x.c

@@ -860,7 +860,8 @@ static irqreturn_t mcp251x_can_ist(int irq, void *dev_id)
 			if (new_state >= CAN_STATE_ERROR_WARNING &&
 			    new_state <= CAN_STATE_BUS_OFF)
 				priv->can.can_stats.error_warning++;
-		case CAN_STATE_ERROR_WARNING:	/* fallthrough */
+			/* fall through */
+		case CAN_STATE_ERROR_WARNING:
 			if (new_state >= CAN_STATE_ERROR_PASSIVE &&
 			    new_state <= CAN_STATE_BUS_OFF)
 				priv->can.can_stats.error_passive++;

drivers/net/can/usb/peak_usb/pcan_usb.c

@@ -415,7 +415,7 @@ static int pcan_usb_decode_error(struct pcan_usb_msg_context *mc, u8 n,
 			new_state = CAN_STATE_ERROR_WARNING;
 			break;
 		}
-		/* else: fall through */
+		/* fall through */
 
 	case CAN_STATE_ERROR_WARNING:
 		if (n & PCAN_USB_ERROR_BUS_HEAVY) {