wl12xx: Include OFDM rates in IBSS mode

We were including only 11b rates in IBSS mode.  This patch adds OFDM
rates.

[Rephrased commit log and removed one unnecessary comment. -- Luca]

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