mac80211: make mpath path fixing more robust

A fixed mpath was not quite being treated as such:

1) if a PERR frame was received, a fixed mpath was
   deactivated.

2) queued path discovery for fixed mpath was potentially
   being considered, changing mpath state.

3) other mpath flags were potentially being inherited when
   fixing the mpath. Just assign PATH_FIXED and SN_VALID.

This solves several issues when fixing a mesh path in one
direction. The reverse direction mpath should probably
also be fixed, or root announcements at least be enabled.

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