wusb: whci-hcd shouldn't do ASL/PZL updates while channel is inactive

ASL/PZL updates while the WUSB channel is inactive (i.e., the PZL and
ASL are stopped) may not complete.  This causes hangs when removing the
whci-hcd module if a device is still connected (removing the device
does an endpoint_disable which results in an ASL update to remove the
qset).

If the WUSB channel is inactive the update can simply be skipped as the
WHC doesn't care about the state of the ASL/PZL.

Signed-off-by: David Vrabel <david.vrabel@csr.com>