hostapd: fix sta psk index for dynamic psk auth

hostapd: fix sta psk index for dynamic psk auth

Depending on the config / circumstances, the get_psk call can be called
multiple times from differnt places, which can lead to wrong sta->psk_idx
values. The correct call is the one that is also interested in the vlan_id,
so use the vlan_id pointer as indication of when to set sta->psk_idx.
Also fix off-by-one error for secondary PSKs

Fixes: b2a2c286170d ("hostapd: add support for authenticating with multiple PSKs via ubus helper")
Signed-off-by: Felix Fietkau <nbd@nbd.name>