libertas: sysfs interface for accessing default mesh channel

This will create the following entry:
/sys/class/net/mshX
            -- boot_options
            |   |-- ...
            |   `-- channel
            ...

... which I overlooked on my previous patch.

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>