can: bittiming: add CAN_KBPS, CAN_MBPS and CAN_MHZ macros

Add three macro to simplify the readability of big bit timing numbers:
  - CAN_KBPS: kilobits per second (one thousand)
  - CAN_MBPS: megabits per second (one million)
  - CAN_MHZ: megahertz per second (one million)

Example:
	u32 bitrate_max = 8 * CAN_MBPS;
	struct can_clock clock = {.freq = 80 * CAN_MHZ};
instead of:
	u32 bitrate_max = 8000000;
	struct can_clock clock = {.freq = 80000000};

Apply the new macro to driver/net/can/dev/bittiming.c.

Link: https://lore.kernel.org/r/20210306054040.76483-1-mailhol.vincent@wanadoo.fr
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev/bittiming.c

@@ -81,9 +81,9 @@ int can_calc_bittiming(struct net_device *dev, struct can_bittiming *bt,
 	if (bt->sample_point) {
 		sample_point_nominal = bt->sample_point;
 	} else {
-		if (bt->bitrate > 800000)
+		if (bt->bitrate > 800 * CAN_KBPS)
 			sample_point_nominal = 750;
-		else if (bt->bitrate > 500000)
+		else if (bt->bitrate > 500 * CAN_KBPS)
 			sample_point_nominal = 800;
 		else
 			sample_point_nominal = 875;

include/linux/can/bittiming.h

@@ -11,6 +11,14 @@
 
 #define CAN_SYNC_SEG 1
 
+
+/* Kilobits and Megabits per second */
+#define CAN_KBPS 1000UL
+#define CAN_MBPS 1000000UL
+
+/* Megahertz */
+#define CAN_MHZ 1000000UL
+
 /*
  * struct can_tdc - CAN FD Transmission Delay Compensation parameters
  *