mac80211: mesh: decrease max drift

The old value was 30ms, which means mesh sync will treat
any value below as merely TSF drift. This isn't really
reasonable (typical drift is < 10us/s) since people
probably want to adjust TSF in smaller increments (for ie.
beacon collision avoidance) without mesh sync fighting
back.

Change max drift adjustment to 0.8ms, so manual TSF
adjustments can be made in 1ms increments, with some
margin.

Signed-off-by: Thomas Pedersen <twp@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>