net: ti: cpsw: enable 10Mbps link speed support in rgmii mode

According to TRMs the 10Mbps link speed is supported in RGMII only when
CPSW2G MAC SL is configured for External Control ("in band") mode
CPSW_SL_MACCTRL.EXT_EN(18) = 1.

Hence update cpsw_slave_update_link() to follow documentation.

[1] https://patchwork.kernel.org/patch/10285239/
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
diff --git a/drivers/net/ti/cpsw.c b/drivers/net/ti/cpsw.c
index 20ddb44..f0d008f 100644
--- a/drivers/net/ti/cpsw.c
+++ b/drivers/net/ti/cpsw.c
@@ -33,6 +33,7 @@
 #define GIGABITEN		BIT(7)
 #define FULLDUPLEXEN		BIT(0)
 #define MIIEN			BIT(15)
+#define CTL_EXT_EN		BIT(18)
 /* DMA Registers */
 #define CPDMA_TXCONTROL		0x004
 #define CPDMA_RXCONTROL		0x014
@@ -489,6 +490,8 @@
 			mac_control |= FULLDUPLEXEN;
 		if (phy->speed == 100)
 			mac_control |= MIIEN;
+		if (phy->speed == 10 && phy_interface_is_rgmii(phy))
+			mac_control |= CTL_EXT_EN;
 	}
 
 	if (mac_control == slave->mac_control)