net: phy: at803x: move qca83xx specific check in dedicated functions

Rework qca83xx specific check to dedicated function to tidy things up
and drop useless phy_id check.

Also drop an useless link_change_notify for QCA8337 as it did nothing an
returned early.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>