wlcore: wait for command completion event when sending CMD_ROLE_STOP

We need to wait for the command completion event when we send the
CMD_ROLE_STOP event otherwise we may try to send CMD_ROLE_START too
soon and get out-of-sync with the firmware.

In some cases, the firmware may not send the event, so we wait for the
event or for the timeout, whichever comes first.

This patch is based on an earlier version by Eliad.

Cc: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>