mac80211: conditionally include timestamp in radiotap information

This makes mac80211 include the low-level MAC timestamp
in the radiotap header if the driver indicated (by a new
RX flag) that the timestamp is valid.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>