net: dsa/slave: Fix compilation warnings

Currently when none of CONFIG_NET_DSA_TAG_DSA, CONFIG_NET_DSA_TAG_EDSA and
CONFIG_NET_DSA_TAG_TRAILER is defined, we get following compilation warnings:

net/dsa/slave.c:51:12: warning: 'dsa_slave_init' defined but not used [-Wunused-function]
net/dsa/slave.c:60:12: warning: 'dsa_slave_open' defined but not used [-Wunused-function]
net/dsa/slave.c:98:12: warning: 'dsa_slave_close' defined but not used [-Wunused-function]
net/dsa/slave.c:116:13: warning: 'dsa_slave_change_rx_flags' defined but not used [-Wunused-function]
net/dsa/slave.c:127:13: warning: 'dsa_slave_set_rx_mode' defined but not used [-Wunused-function]
net/dsa/slave.c:136:12: warning: 'dsa_slave_set_mac_address' defined but not used [-Wunused-function]
net/dsa/slave.c:164:12: warning: 'dsa_slave_ioctl' defined but not used [-Wunused-function]

Earlier approach to fix this was discussed here:

lkml.org/lkml/2012/10/29/549

This is another approach to fix it. This is done by some changes in config
options, which make more sense than the earlier approach. As, atleast one
tagging option must always be selected for using net/dsa/ infrastructure, this
patch selects NET_DSA from tagging configs instead of having it as an selectable
config.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>