[MPC512x] Streamline frame handling in the FEC driver

- convert frame size settings to be derived from a single base
- set frame size to the recommended default value

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>
diff --git a/cpu/mpc512x/fec.h b/cpu/mpc512x/fec.h
index d2d877a..9c38502 100644
--- a/cpu/mpc512x/fec.h
+++ b/cpu/mpc512x/fec.h
@@ -164,10 +164,13 @@
 #define FEC_RBD_NUM		32	/* The user can adjust this value */
 
 /* packet size limit */
-#define FEC_MAX_PKT_SIZE	1536
+#define FEC_MAX_FRAME_LEN	1522	/* recommended default value */
+
+/* Buffer size must be evenly divisible by 16 */
+#define FEC_BUFFER_SIZE		((FEC_MAX_FRAME_LEN + 0x10) & (~0xf))
 
 typedef struct {
-	uint8 frame[FEC_MAX_PKT_SIZE];
+	uint8 frame[FEC_BUFFER_SIZE];
 } mpc512x_frame;
 
 typedef struct {