net: ethernet: stmmac: Fix signedness bug in ipq806x_gmac_of_parse()

The "gmac->phy_mode" variable is an enum and in this context GCC will
treat it as an unsigned int so the error handling will never be
triggered.

Fixes: b1c17215 ("stmmac: add ipq806x glue layer")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>