cfg80211: Use consistent BSS matching between scan and sme

cfg80211 scan code adds separate BSS entries if the same BSS shows up
on multiple channels. However, sme implementation does not use the
frequency when fetching the BSS entry. Fix this by adding channel
information to cfg80211_roamed() and include it in cfg80211_get_bss()
calls.

Please note that drivers using cfg80211_roamed() need to be modified to
fully implement this fix. This commit includes only minimal changes to
avoid compilation issues; it maintains the old (broken) behavior for
most drivers. ath6kl was the only one that I could test, so I updated
it to provide the operating frequency in the roamed event.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>