wlcore: get channel from bss_conf instead of hw->conf

We care only about the operational channel, not
about the temporal hw channel (which won't have
any real meaning in multi-channel env anyway)

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