net: dsa: do not cast dst

dsa_ptr is not a void pointer anymore since Nov 2011, as of cf50dcc2
("dsa: Change dsa_uses_{dsa, trailer}_tags() into inline functions"),
but an explicit dsa_switch_tree pointer, thus remove the (void *) cast.

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