can: flexcan: replace open coded "mailbox code" by proper define

This patch replaces a open coded variant of a "mailbox code" definition by an
existing define, improves code readability.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/flexcan.c

@@ -158,7 +158,6 @@
 	 FLEXCAN_IFLAG_BUF(FLEXCAN_TX_BUF_ID))
 
 /* FLEXCAN message buffers */
-#define FLEXCAN_MB_CNT_CODE(x)		(((x) & 0xf) << 24)
 #define FLEXCAN_MB_CODE_RX_INACTIVE	(0x0 << 24)
 #define FLEXCAN_MB_CODE_RX_EMPTY	(0x4 << 24)
 #define FLEXCAN_MB_CODE_RX_FULL		(0x2 << 24)
@@ -478,7 +477,7 @@ static int flexcan_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct flexcan_regs __iomem *regs = priv->base;
 	struct can_frame *cf = (struct can_frame *)skb->data;
 	u32 can_id;
-	u32 ctrl = FLEXCAN_MB_CNT_CODE(0xc) | (cf->can_dlc << 16);
+	u32 ctrl = FLEXCAN_MB_CODE_TX_DATA | (cf->can_dlc << 16);
 
 	if (can_dropped_invalid_skb(dev, skb))
 		return NETDEV_TX_OK;