net: dsa: Drop WARN() in tag_brcm.c

We may be able to see invalid Broadcom tags when the hardware and drivers are
misconfigured, or just while exercising the error path. Instead of flooding
the console with messages, flat out drop the packet.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>