net: phy: dp83869: fix mii mode when rgmii strap cfg is used

The DP83869 PHY on TI k3-am642-evm is supported both MII and RGMII
interfaces and configured by default to use RGMII interface (strap), but
the board design is also allowed to switch dynamically to MII interface for
testing purposes by applying different set of pinmuxes.

To support such switch to MII interface update the DP83869 PHY driver to
configure OP_MODE_DECODE.RGMII_MII_SEL(bit 5) properly when MII PHY
interface mode is requested.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>