staging: wlan-ng: correction to comment in hfa384x_usb

The comment for hfa384x_docmd incorrectly states that usercb_data
should be NULL for DOASYNC calls; in fact, it should be NULL for
DOWAIT calls (this is consistent with the other similar functions and
the rest of the comment text).

Signed-off-by: Tim Collier <osdevtc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>