[realtek] Use explicit value for TCR.MXDMA

Some RTL8169 cards (observed with an RTL8169SC) power up with
TCR.MXDMA set to 16 bytes.  While this does not prevent proper
operation, it almost certainly degrades performance.

Fix by explicitly setting TCR.MXDMA to "unlimited".

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/realtek.c

  */
 static int realtek_open ( struct net_device *netdev ) {
 	struct realtek_nic *rtl = netdev->priv;
+	uint32_t tcr;
 	uint32_t rcr;
 	int rc;
 
 	 */
 	writeb ( ( RTL_CR_TE | RTL_CR_RE ), rtl->regs + RTL_CR );
 
+	/* Configure transmitter */
+	tcr = readl ( rtl->regs + RTL_TCR );
+	tcr &= ~RTL_TCR_MXDMA_MASK;
+	tcr |= RTL_TCR_MXDMA_DEFAULT;
+	writel ( tcr, rtl->regs + RTL_TCR );
+
 	/* Configure receiver */
 	rcr = readl ( rtl->regs + RTL_RCR );
 	rcr &= ~( RTL_RCR_RXFTH_MASK | RTL_RCR_RBLEN_MASK |

src/drivers/net/realtek.h

 /** Interrupt Status Register (word) */
 #define RTL_ISR 0x3e
 
+/** Transmit (Tx) Configuration Register (dword) */
+#define RTL_TCR 0x40
+#define RTL_TCR_MXDMA(x)	( (x) << 8 ) /**< Max DMA burst size */
+#define RTL_TCR_MXDMA_MASK	RTL_TCR_MXDMA ( 0x7 )
+#define RTL_TCR_MXDMA_DEFAULT	RTL_TCR_MXDMA ( 0x7 /* Unlimited */ )
+
 /** Receive (Rx) Configuration Register (dword) */
 #define RTL_RCR 0x44
 #define RTL_RCR_RXFTH(x)	( (x) << 13 ) /**< Receive FIFO threshold */