net: phy: at803x: select correct page on config init

The Atheros AR8031 and AR8033 expose different registers for SGMII/Fiber
as well as the copper side of the PHY depending on the BT_BX_REG_SEL bit
in the chip configure register.

The driver assumes the copper side is selected on probe, but this might
not be the case depending which page was last selected by the
bootloader. Notably, Ubiquiti UniFi bootloaders show this behavior.

Select the copper page when probing to circumvent this.

Signed-off-by: David Bauer <mail@david-bauer.net>
Signed-off-by: David S. Miller <davem@davemloft.net>