libertas: Extend MESH_CONFIG command to access non-volatile configuration

This patch is based on a patch from Shailendra Govardhan and Brian Cavagnolo.
It extends the MESH_CONFIG command to configure non-volatile parameters on
libertas devices that support them (e.g. OLPC Active Antenna).

This patch only implements the driver/firmware interface.

See http://dev.laptop.org/ticket/6823

 for minimal testing results and known
issues.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>