libertas: clean up direct command handling

Move direct command handling through __lbs_cmd() over to using the
header as the first member of the command structure, and only define
the __lbs_cmd() callback in one place rather than 3.  Convert boot2
version command to new usage.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>