net: dsa: rework switch addition and removal

This patch removes the unnecessary index argument from the
dsa_dst_add_ds and dsa_dst_del_ds functions and renames them to
dsa_tree_add_switch and dsa_tree_remove_switch respectively.

In addition to a more explicit scope, we now check the presence of an
existing switch with the same index directly within dsa_tree_add_switch.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>