mt76: mt7915: fix calling mt76_wcid_alloc with incorrect parameter

It will cause maximum connectable STA to be one less
when calling mt76_wcid_alloc with parameter MT7915_WTBL_STA - 1.

Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Sujuan Chen <sujuan.chen@mediatek.com>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>