libertas: make a handy lbs_cmd_async() command

This uses a static lbs_cmd_async_callback function, which is a
noop. Just setting the callback argument to __lbs_cmd_async()
to NULL won't work, because then the cmdnode wouldn't be
released.

This also makes __lbs_cmd_async() a static method, which is
now only used by lbs_cmd() and lbs_cmd_async().

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>