P2P: Allow P2P client to specify preferred group channel

P2P: Allow P2P client to specify preferred group channel

When re-invoking a persistent group in P2P client role, the new
pref=<MHz> parameter can now be used with the p2p_invite command to
indicate a preferred operating frequency. Unlike the older freq=<MHz>
parameter, this leaves GO an option to select another channel (from our
supported channels) if the GO cannot accept the channel.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>