Set the beacon timestamp once during "transmission" so the
monitor interface also gets a timestamped beacon.

Also use a common base between TX timestamp and RX
mactime. This eliminates "TX" path delay, which shows up
as a constant error in Toffset.

Get the global TSF once before iterating over all RX HWs,
so they all set a mactime with the same time base.
Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>