net: designware: Respect "bus mode" register contents on SW reset

"bus mode" register contains lots of fields and some of them don't
expect to be written with 0 (zero). So since we're only interested in
resetting MAC (which is done with setting the least significant bit of
this register with "0") I believe it's better to modify only 1 bit of
the register.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Vipin Kumar <vipin.kumar@st.com>
Patch: 277864

drivers/net/designware.c

@@ -96,7 +96,7 @@ static int mac_reset(struct eth_device *dev)
 	ulong start;
 	int timeout = CONFIG_MACRESET_TIMEOUT;
 
-	writel(DMAMAC_SRST, &dma_p->busmode);
+	writel(readl(&dma_p->busmode) | DMAMAC_SRST, &dma_p->busmode);
 
 	if (priv->interface != PHY_INTERFACE_MODE_RGMII)
 		writel(MII_PORTSELECT, &mac_p->conf);