USB: Export URB statistics for powertop

powertop currently tracks interrupts generated by uhci, ehci, and ohci,
but it has no way of telling which USB device to blame USB bus activity on.
This patch exports the number of URBs that are submitted for a given device.
Cat the file 'urbnum' in /sys/bus/usb/devices/.../

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