dsa: tag_brcm: Avoid unused symbols

It is possible that the driver is compiled with both
CONFIG_NET_DSA_TAG_BRCM and CONFIG_NET_DSA_TAG_BRCM_PREPEND disabled.
This results in warnings about unused symbols. Add some conditional
compilation to avoid this.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>

v2
Reorder patch to before tag drivers can be modules

Signed-off-by: David S. Miller <davem@davemloft.net>