net: stmmac: Don't set has_gmac if has_gmac4 is set

Some Rockchip platforms have a GMAC4 core, and therefore
'plat_stmmacenet_data.has_gmac' shouldn't be set if
'plat_stmmacenet_data.has_gmac4' is set.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>