net: ethernet: ti: cpsw: add check for in-band mode setting with RGMII PHY interface

According to AM335x TRM[1] 14.3.6.2, AM437x TRM[2] 15.3.6.2 and
DRA7 TRM[3] 24.11.4.8.7.3.3, in-band mode in EXT_EN(bit18) register is only
available when PHY is configured in RGMII mode with 10Mbps speed. It will
cause some networking issues without RGMII mode, such as carrier sense
errors and low throughput. TI also mentioned this issue in their forum[4].

This patch adds the check mechanism for PHY interface with RGMII interface
type, the in-band mode can only be set in RGMII mode with 10Mbps speed.

References:
[1]: https://www.ti.com/lit/ug/spruh73p/spruh73p.pdf
[2]: http://www.ti.com/lit/ug/spruhl7h/spruhl7h.pdf
[3]: http://www.ti.com/lit/ug/spruic2b/spruic2b.pdf
[4]: https://e2e.ti.com/support/arm/sitara_arm/f/791/p/640765/2392155Suggested-by: NHolsety Chen (陳憲輝) <Holsety.Chen@moxa.com>
Signed-off-by: NSZ Lin (林上智) <sz.lin@moxa.com>
Signed-off-by: NSchuyler Patton <spatton@ti.com>
Reviewed-by: NGrygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/ti/cpsw.c

@@ -1014,7 +1014,8 @@ static void _cpsw_adjust_link(struct cpsw_slave *slave,
 		/* set speed_in input in case RMII mode is used in 100Mbps */
 		if (phy->speed == 100)
 			mac_control |= BIT(15);
-		else if (phy->speed == 10)
+		/* in band mode only works in 10Mbps RGMII mode */
+		else if ((phy->speed == 10) && phy_interface_is_rgmii(phy))
 			mac_control |= BIT(18); /* In Band mode */
 
 		if (priv->rx_pause)