Convert CONFIG_SYS_RX_ETH_BUFFER to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_RX_ETH_BUFFER

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/include/net.h b/include/net.h
index b02e4f6..675bf41 100644
--- a/include/net.h
+++ b/include/net.h
@@ -35,13 +35,7 @@
  *	alignment in memory.
  *
  */
-
-#ifdef CONFIG_SYS_RX_ETH_BUFFER
-# define PKTBUFSRX	CONFIG_SYS_RX_ETH_BUFFER
-#else
-# define PKTBUFSRX	4
-#endif
-
+#define PKTBUFSRX	CONFIG_SYS_RX_ETH_BUFFER
 #define PKTALIGN	ARCH_DMA_MINALIGN
 
 /* Number of packets processed together */