net: stmmac: set get_rx_header_len() as void for it didn't have any error code to return

We found the following warning when using W=1 to build kernel:

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:3634:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
int ret, coe = priv->hw->rx_csum;

When digging stmmac_get_rx_header_len(), dwmac4_get_rx_header_len() and
dwxgmac2_get_rx_header_len() return 0 only, without any error code to
report. Therefore, it's better to define get_rx_header_len() as void.

Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>