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>

drivers/net/ethernet/stmicro/stmmac/hwif.h

@@ -564,7 +564,6 @@ struct stmmac_mode_ops {
 #define stmmac_clean_desc3(__priv, __args...) \
 	stmmac_do_void_callback(__priv, mode, clean_desc3, __args)
 
-struct stmmac_priv;
 struct tc_cls_u32_offload;
 struct tc_cbs_qopt_offload;
 struct flow_cls_offload;