mesh_mpm: don't check the WLAN_STA_AUTH flag for non secure mesh

For non secure mesh, avoid checking the WLAN_STA_AUTH flag
before inserting the STA.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Merge pull request #8 from cozybit/yeow-fix-user_mpm

mesh_mpm: don't check the WLAN_STA_AUTH flag for non secure mesh