dsa: simplify Kconfig symbols and dependencies

1. Remove CONFIG_HAVE_NET_DSA.

CONFIG_HAVE_NET_DSA is a legacy leftover from the times when drivers
should have selected CONFIG_NET_DSA manually.
Currently, all drivers has explicit 'depends on NET_DSA', so this is
no more needed.

2. CONFIG_HAVE_NET_DSA dependencies became CONFIG_NET_DSA's ones.

 - dropped !S390 dependency which was introduced to be sure NET_DSA
   can select CONFIG_PHYLIB. DSA migrated to Phylink almost 3 years
   ago and the PHY library itself doesn't depend on !S390 since
   commit 870a2b5e

 ("phylib: remove !S390 dependeny from Kconfig");
 - INET dependency is kept to be sure we can select NET_SWITCHDEV;
 - NETDEVICES dependency is kept to be sure we can select PHYLINK.

3. DSA drivers menu now depends on NET_DSA.

Instead on 'depends on NET_DSA' on every single driver, the entire
menu now depends on it. This eliminates a lot of duplicated lines
from Kconfig with no loss (when CONFIG_NET_DSA=m, drivers also can
be only m or n).
This also has a nice side effect that there's no more empty menu on
configurations without DSA.

4. Kbuild will now descend into 'drivers/net/dsa' only when
   CONFIG_NET_DSA is y or m.

This is safe since no objects inside this folder can be built without
DSA core, as well as when CONFIG_NET_DSA=m, no objects can be
built-in.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>