wlcore: allow only the lowest OFDM rate for p2p setup frames

The IEEE80211_TX_CTL_NO_CCK_RATE flag is only set for mgmt packets
transmitted during p2p connection setup. Make sure to use the lowest
OFDM rate guarantee the peer always hears us.
Change the p2p rate policy to contain only the 6mpbs rate to acheive
this effect.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>