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>

include/net.h

@@ -35,13 +35,7 @@ struct udevice;
  *	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 */