mt76x2u: init: use common routines for wcid/key initialization

Use mt76x02_mac_wcid_setup and mt76x02_mac_shared_key_setup for
shared keys and wcid table initialization and remove duplicated
code

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>