mesh: Create new station entry on popen frames

mesh: Create new station entry on popen frames

Currently, there is a race in open mesh networks where mesh STA A
receives a beacon from B and sends a peering open frame to initiate
peering. STA B, having not yet received a beacon from A and thus
created the corresponding station entry, will ignore all such open
frames. If the beacon interval is sufficiently long then peering
will not succeed as a result.

In fact B can simply create the station entry when the popen is
received, as is done in Linux's in-kernel MPM, avoiding the issue.

Signed-off-by: Bob Copeland <me@bobcopeland.com>