net: mscc: ocelot: don't use magic numbers for OCELOT_POLICER_DISCARD

OCELOT_POLICER_DISCARD helps "kill dropped packets dead" since a
PERMIT/DENY mask mode with a port mask of 0 isn't enough to stop the CPU
port from receiving packets removed from the forwarding path.

The hardcoded initialization done for it in ocelot_vcap_init() is
confusing. All we need from it is to have a rate and a burst size of 0.

Reuse qos_policer_conf_set() for that purpose.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>