net: mscc: ocelot: use list_add_tail in ocelot_vcap_filter_add_to_block()

list_add(..., pos->prev) and list_add_tail(..., pos) are equivalent, use
the later form to unify with the case where the list is empty later.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>