wifi: mt76: mt7996: prevent uninit return in mt7996_mac_sta_add_links

wifi: mt76: mt7996: prevent uninit return in mt7996_mac_sta_add_links

If link_conf_dereference_protected() or mt7996_vif_link()
or link_sta_dereference_protected() fail the code jumps to
the error_unlink label and returns ret which is uninitialised.

Fix this by setting err before jumping to error_unlink.

Fixes: c7e4fc362443 ("wifi: mt76: mt7996: Update mt7996_mcu_add_sta to MLO support")
Fixes: dd82a9e02c05 ("wifi: mt76: mt7996: Rely on mt7996_sta_link in sta_add/sta_remove callbacks")
Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
Link: https://patch.msgid.link/20250421110550.9839-1-qasdev00@gmail.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>