can: at91_can: directly define AT91_MB_RX_LAST

...instead of deriving it from AT91_MB_RX_FIRST and AT91_MB_RX_NUM.
This removes a level of computation, when switching the driver from
compile time constants to runtime values.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/at91_can.c

@@ -47,11 +47,10 @@
  * RX/TX Mailbox split
  * don't dare to touch
  */
-#define AT91_MB_RX_NUM		11
 #define AT91_MB_TX_SHIFT	2
 
 #define AT91_MB_RX_FIRST	1
-#define AT91_MB_RX_LAST		(AT91_MB_RX_FIRST + AT91_MB_RX_NUM - 1)
+#define AT91_MB_RX_LAST		11
 
 #define AT91_MB_RX_MASK(i)	((1 << (i)) - 1)
 #define AT91_MB_RX_SPLIT	8