wlcore: use link count for single-STA-PSM optimization

Only allow a PSM STA to congest FW memory when it is the single active
link. Being a single STA doesn't imply a single link - there might be
other links on other roles.

[Changed WARN_ON to WARN_ON_ONCE -- Luca]

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>