net: phy: DP83822: enable rgmii mode if phy_interface_is_rgmii

RGMII mode can be enable from dp83822 straps, and also writing bit 9
of register 0x17 - RMII and Status Register (RCSR).
When phy_interface_is_rgmii rgmii mode must be enabled, same for
contrary, this prevents malconfigurations of hw straps

References:
 - https://www.ti.com/lit/gpn/dp83822i

 p66

Signed-off-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>
Co-developed-by: Michael Trimarchi <michael@amarulasolutions.com>
Suggested-by: Alberto Bianchi <alberto.bianchi@amarulasolutions.com>
Tested-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>