cfg80211: Clean up connect params and channel fetching

Addition of the frequency hints showed up couple of places in cfg80211
where pointers could be marked const and a shared function could be used
to fetch a valid channel.

Signed-off-by: Jouni Malinen <j@w1.fi>
[fix mwifiex]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --cc drivers/net/wireless/mwifiex/cfg80211.c
Simple merge

diff --cc include/net/cfg80211.h
Simple merge

diff --cc net/wireless/nl80211.c
Simple merge