xhci: If no endpoints changed, don't issue BW command.

Some alternate interface settings have no endpoints associated with them.
This shows up in some USB webcams, particularly the Logitech HD 1080p,
which uses the uvcvideo driver.  If a driver switches between two alt
settings with no endpoints, there is no need to issue a configure endpoint
command, because there is no endpoint information to update.

The only time a configure endpoint command with just the add slot flag set
makes sense is when the driver is updating hub characteristics in the slot
context.  However, that code never calls xhci_check_bandwidth, so we
should be safe not issuing a command if only the slot context add flag is
set.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>