net: dsa: make NET_DSA manually selectable from the config

Change bd76a116 made all DSA drivers
depend on NET_DSA rather than selecting them. However, as the only way
to select this option was to actually select a driver, it made DSA
impossible to enable at all.

This patch adds an explicit entry which the user will have to enable
prior selecting a driver.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>