net: stmmac: pci: remove the duplicate code of set phy_mask

All members of mdio_bus_data are cleared to 0 when it was obtained
by devm_kzalloc(). so It doesn't need to set phy_mask as 0 again.

Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>