Move specific at8031 WOL enable/disable to dedicated function to make
at803x_set_wol more generic.

This is needed in preparation for PHY driver split as qca8081 share the
same function to toggle WOL settings.

In this new implementation WOL module in at8031 is enabled after the
generic interrupt is setup. This should not cause any problem as the
WOL_INT has a separate implementation and only relay on MAC bits.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>