libertas: don't use __lbs_cmd() with empty callback in if_usb.c

We're about to change semantics, leaving callers of
lbs_prepare_and_send_command() with the old broken priv->cur_cmd_retcode
crap. The existence of the callback command will be the trigger for the
new semantics when handling the response.

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