[forcedeth] Increase TX ring size to prevent dropped packets

Commit 947976d ("[netdevice] Do not force a poll on net_tx()")
requires network devices to have TX rings that are sufficiently large
to allow a transmitted response to all packets received during a
single poll.

Reported-by: Robin Smidsrød <robin@smidsrod.no>
Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/forcedeth.h

 #define DESC_VER_3	3
 
 #define RX_RING_SIZE		16
-#define TX_RING_SIZE		16
+#define TX_RING_SIZE		32
 #define RXTX_RING_SIZE		( ( RX_RING_SIZE ) + ( TX_RING_SIZE ) )
 #define RX_RING_MIN		128
 #define TX_RING_MIN		64