wusbcore: fix root hub hub_status_data to only return > 0 if status has actually changed

The hub_status_data function on the wireless USB root hub controller
(wusbhc_rh_status_data) always returns a positive value even if no ports
have changed.  This patch updates wusbhc_rh_status_data to only return a
positive value if the root hub status needs to be queried.  The current
implementation can also leave the upper bits of the port bitmap
uninitialized if wusbhc->ports_max is not one less than an even multiple
of 8.  This patch fixes that as well by initializing the buffer to 0.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>