mac80211: rename bss_conf timestamp to last_tsf

This value is not really very useful by itself,
yet some drivers (including iwlwifi until I can
figure out what it should do) use it. At least
rename it to "last_tsf" to indicate the meaning
and add a note that it may be really old.

I suspect the value may become useful combined
with the rx_status->mactime, but we don't (yet)
store that value and pass it to the driver.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>