wl12xx: configure rates when working in ibss mode

When working in ibss mode, we don't configure rate policy per station
(as we use the same link for multiple stations), so currently the
1mb/s rate is being used.

Instead, configure the firmware to use the whole 11b rates by default.

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