nios2: nios2-generic: do not allocate rx buf in net.c

Do not allocate rx buf in net.c, because altera_tse allocates
its own rx buf in driver. This can save 6KB memory.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>

include/configs/nios2-generic.h

@@ -33,8 +33,9 @@
 #define CONFIG_SYS_MAX_FLASH_SECT	512
 
 /*
- * MII/PHY
+ * NET options
  */
+#define CONFIG_SYS_RX_ETH_BUFFER	0
 #define CONFIG_CMD_MII
 #define CONFIG_PHY_GIGE
 #define CONFIG_SYS_FAULT_ECHO_LINK_DOWN