libertas: add opaque extra argument to cmd callback function

libertas: add opaque extra argument to cmd callback function

This will be useful for letting callbacks do stuff like copying the
response into a buffer provided by the caller of lbs_cmd()

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