net: phy: microchip_t1s: replace read-modify-write code with phy_modify_mmd

Replace read-modify-write code in the lan867x_config_init function to
avoid handling data type mismatch and to simplify the code.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Parthiban Veerasooran <Parthiban.Veerasooran@microchip.com>
Reviewed-by: Ramón Nordin Rodriguez <ramon.nordin.rodriguez@ferroamp.se>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>