net/designware: add explicit reset of {tx|rx}_currdescnum

Driver "init" function might be called multiple times.
On every "init" Tx/Rx buffer descriptors are initialized: "descs_init"
-> "{tx|rx}_descs_init".

In its turn those init functions set MAC's "{tx|rx}desclistaddr" to
point on the first buffer descriptor in the list.

So CPU to start operation from the first buffer descriptor as well after
every "init" we have to reset "{tx|rx}_currdescnum".

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Vipin Kumar <vipin.kumar@st.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/drivers/net/designware.c b/drivers/net/designware.c
index 22155b4..1868089 100644
--- a/drivers/net/designware.c
+++ b/drivers/net/designware.c
@@ -52,6 +52,7 @@
 	desc_p->dmamac_next = &desc_table_p[0];
 
 	writel((ulong)&desc_table_p[0], &dma_p->txdesclistaddr);
+	priv->tx_currdescnum = 0;
 }
 
 static void rx_descs_init(struct eth_device *dev)
@@ -79,6 +80,7 @@
 	desc_p->dmamac_next = &desc_table_p[0];
 
 	writel((ulong)&desc_table_p[0], &dma_p->rxdesclistaddr);
+	priv->rx_currdescnum = 0;
 }
 
 static void descs_init(struct eth_device *dev)