net: usb: asix: Fix less than zero comparison of a u16

The comparison of the u16 priv->phy_addr < 0 is always false because
phy_addr is unsigned. Fix this by assigning the return from the call
to function asix_read_phy_addr to int ret and using this for the
less than zero error check comparison.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: e532a096 ("net: usb: asix: ax88772: add phylib support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>