net: dsa: bcm_sf2: dst in not an array

It's been a while now since ds->dst is not an array anymore, but a
simple pointer to a dsa_switch_tree.

Fortunately, SF2 does not support multi-chip and thus ds->index is
always 0.

This patch substitutes 'ds->dst[ds->index].' with 'ds->dst->'.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>