p2p packets should go out only with OFDM rates.

Configure a new rate policy that will (later) be used
during p2p_find (when the p2p_cli / p2p_go interfaces
are in use, we won't have to use this policy, as
the configured rates should already be OFDM-only).

Additionally, update CONF_TX_MAX_RATE_CLASSES to reflect
the current value from the fw api.
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>