wlcore: fix wrong remote rates when starting STA role

In wl18xx, we use a new ACX command in order to set the remote
supported rates, once we know it (ie. after association).  The wl12xx
firmware doesn't support changing the rates after the STA is started,
so we need to use all supported rates.

Commit 530abe19 (wlcore: add ACX_PEER_CAP command) broke that by using
wlvif->rate_set when starting the STA role.

Signed-off-by: Luciano Coelho <coelho@ti.com>