libertas: Add libertas_disablemesh module parameter to disable mesh interface

This allows individual users and deployments to disable mesh support at
runtime, i.e. without having to build and maintain a custom kernel.

Based on a patch by Paul Fox <pgf@laptop.org>.
Signed-off-by: Sascha Silbe <silbe@activitycentral.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>