net: dsa: tag_brcm: Fix skb->fwd_offload_mark location

When the backport of upstream commit
0e62f543bed03a64495bd2651d4fe1aa4bcb7fe5 ("net: dsa: Fix duplicate
frames flooded by learning") was done the assignment of
skb->fwd_offload_mark would land in brcm_tag_xmit() which is incorrect,
it should have been in brcm_tag_rcv().

Fixes: 5e845dc6 ("net: dsa: Fix duplicate frames flooded by learning")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>