net: stmmac: Remove duplicate declaration of stmmac_priv

In commit f4da5652 ("net: stmmac: Add support for external
trigger timestamping"), struct stmmac_priv was declared at line 507
which caused duplicate struct declarations.
Remove later duplicate declaration here.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>