net: mscc: ocelot: use list_for_each_entry in ocelot_vcap_filter_add_to_block

Unify the code paths for adding to an empty list and to a list with
elements by keeping a "pos" list_head element that indicates where to
insert. Initialize "pos" with the list head itself in case
list_for_each_entry() doesn't iterate over any element.

Note that list_for_each_safe() isn't needed because no element is
removed from the list while iterating.

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