net: phy: introduce genphy_c45_pma_baset1_setup_master_slave()

Move baset1 specific part of genphy_c45_pma_setup_forced() code to
separate function to make it reusable by PHY drivers.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>