staging: wlan-ng: Replace data type declaration with variable of same type in hfa384x_usb.c

sizeof(var) instead of sizeof(struct XXX) is preferred.
Fix it in hfa384x_usb.c file.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>