libertas: switch lbs_cmd() to take a callback function pointer

All existing code which sends commands is set up to have some function
called with the results, not to get data back. It's more versatile this
way, and providing it with a callback function which involves memcpy()
is hardly difficult.

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