net: usb: asix: refactor asix_read_phy_addr() and handle errors on return

Refactor asix_read_phy_addr() to return usable error value directly and
make sure all callers handle this error.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>