mlxsw: spectrum_span: Prevent duplicate mirrors

In net commit 8175f7c4736f ("mlxsw: spectrum: Prevent duplicate
mirrors") we prevented the user from mirroring more than once from a
single binding point (port-direction pair).

The fix was essentially reverted in a merge conflict resolution when net
was merged into net-next. Restore it.

Fixes: 03fe2deb ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net"

)
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>