wifi: mt76: mt7921: prevent decap offload config before STA initialization

wifi: mt76: mt7921: prevent decap offload config before STA initialization

The decap offload configuration should only be applied after the STA has
been successfully initialized. Attempting to configure it earlier can lead
to corruption of the MAC configuration in the chip's hardware state.

Add an early check for `msta->deflink.wcid.sta` to ensure the station peer
is properly initialized before proceeding with decapsulation offload
configuration.

Cc: stable@vger.kernel.org
Fixes: 24299fc869f7 ("mt76: mt7921: enable rx header traslation offload")
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Link: https://patch.msgid.link/f23a72ba7a3c1ad38ba9e13bb54ef21d6ef44ffb.1748149855.git.deren.wu@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>