net: phy: at803x: move specific at8031 config_intr to dedicated function

Move specific at8031 config_intr bits to dedicated function to make
at803x_config_initr more generic.

This is needed in preparation for PHY driver split as qca8081 share the
same function to setup interrupts.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>