LinuxKPI: 802.11 handle IEEE80211_CONF_IDLE better

We never initialized hw->conf.flags for IEEE80211_CONF_IDLE but
on set_channel we would clear it and announce a change.
This lead to a problem that drivers may do some work every time
which was not needed and may lead to unexpected behaviour (for no
better driver code).

Properly initialize conf.flags with IEEE80211_CONF_IDLE.
Factor out the toggling into a function and clear IDLE while
sw scanning and when associated and set again when scan ends
or we are bouncing out of assoc.

MFC after: 3 days