mac80211: mesh: only get tsf if we need it

The local TSF timer is used to compute the timing offset between
mesh peers on beacon reception.  However, asking the device for
the TSF is not very accurate, so we prefer to use rx->mactime
if available.  In the latter case, calling drv_get_tsf() just
adds more delay into the RX path, so skip it if we can.

Signed-off-by: Bob Copeland <bob@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>