[realtek] Allow extra space in RX buffers

Some hardware (observed with an onboard RTL8168) will erroneously
report a buffer overflow error if the received packet exactly fills
the receive buffer.

Fix by adding an extra four bytes of padding to each receive buffer.

Debugged-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/realtek.h

 #define RTL_NUM_RX_DESC 4
 
 /** Receive buffer length */
-#define RTL_RX_MAX_LEN ( ETH_FRAME_LEN + 4 /* VLAN */ + 4 /* CRC */ )
+#define RTL_RX_MAX_LEN \
+	( ETH_FRAME_LEN + 4 /* VLAN */ + 4 /* CRC */ + 4 /* extra space */ )
 
 /** A Realtek descriptor ring */
 struct realtek_ring {