libertas: use priv->mesh_tlv instead of priv->mesh_fw_ver

Both variables contained the same information (no mesh, old mesh, new mesh).
So we can get rid of one variable.

Also move the mesh-version test from cmd.c into mesh.c.

Signed-off-by: Holger Schurig <holgerschurig@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>