mac80211: better definition of mactime

define mactime as the time when the first data symbol arrived at the HW. the
old definition was questionable because 802.11 defines timestamp only for
beacon and probe response frames, and there it means the timestamp field.

a stricter definition of mactime is necessary for correct merging of IBSS.

note that it is up to the driver to convert whatever its hardware returns to
this definition. unfortunately we don't know for example when atheros hardware
takes its rx timestamp exactly :(

Signed-off-by: Bruno Randolf <bruno@thinktube.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>