Fix gcc build error:

net/dsa/tag_brcm.c:211:16: error: brcm_prepend_netdev_ops undeclared here (not in a function); did you mean brcm_netdev_ops?
 DSA_TAG_DRIVER(brcm_prepend_netdev_ops);
                ^
./include/net/dsa.h:708:10: note: in definition of macro DSA_TAG_DRIVER
  .ops = &__ops,       \
          ^~~~~
./include/net/dsa.h:701:36: warning: dsa_tag_driver_brcm_prepend_netdev_ops defined but not used [-Wunused-variable]
 #define DSA_TAG_DRIVER_NAME(__ops) dsa_tag_driver ## _ ## __ops
                                    ^
./include/net/dsa.h:707:30: note: in expansion of macro DSA_TAG_DRIVER_NAME
 static struct dsa_tag_driver DSA_TAG_DRIVER_NAME(__ops) = {  \
                              ^~~~~~~~~~~~~~~~~~~
net/dsa/tag_brcm.c:211:1: note: in expansion of macro DSA_TAG_DRIVER
 DSA_TAG_DRIVER(brcm_prepend_netdev_ops);

Like the CONFIG_NET_DSA_TAG_BRCM case,
brcm_prepend_netdev_ops and DSA_TAG_PROTO_BRCM_PREPEND
should be wrappeed by CONFIG_NET_DSA_TAG_BRCM_PREPEND.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: b74b70c4

 ("net: dsa: Support prepended Broadcom tag")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>