USB: fix regression occurring during device removal

This patch (as1476) fixes a regression introduced by
fccf4e86

 (USB: Free bandwidth when
usb_disable_device is called).  usb_disconnect() grabs the
bandwidth_mutex before calling usb_disable_device(), which calls down
indirectly to usb_set_interface(), which tries to acquire the
bandwidth_mutex.

The fix causes usb_set_interface() to return early when it is called
for an interface that has already been unregistered, which is what
happens in usb_disable_device().

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>