cfg80211: unify cfg80211_roamed() and cfg80211_roamed_bss()

cfg80211_roamed() and cfg80211_roamed_bss() take the same arguments
except that cfg80211_roamed() requires the BSSID and
cfg80211_roamed_bss() requires the bss entry.

Unify the two functions by using a struct for driver initiated
roaming information so that either the BSSID or the bss entry can be
passed as an argument to the unified function.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
[modified the ath6k, brcm80211, rndis and wlan-ng drivers accordingly]
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
[modify brcmfmac to remove the useless cast, spotted by Arend]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>