of: mdio: Fix a signedness bug in of_phy_get_and_connect()

The "iface" variable is an enum and in this context GCC treats it as
an unsigned int so the error handling is never triggered.

Fixes: b7862412 ("of_mdio: Abstract a general interface for phy connect")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>