net: stmmac: dwmac-meson8b: Fix signedness bug in probe

The "dwmac->phy_mode" is an enum and in this context GCC treats it as
an unsigned int so the error handling is never triggered.

Fixes: 566e8251 ("net: stmmac: add a glue driver for the Amlogic Meson 8b / GXBB DWMAC")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>