musb: potential use after free

We assign "urb->hcpriv = qh;" a few lines down.  I'm pretty sure we
want it "urb->hcpriv" to be NULL not a freed value.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>