usb: Always return 0 or -EBUSY to the runtime PM core.

The PM core reacts badly when the return code from usb_runtime_suspend()
is not 0, -EAGAIN, or -EBUSY.  The PM core regards this as a fatal error,
and refuses to run anymore PM helper functions.  In particular,
usbfs_open() and other usbfs functions will fail because the PM core will
return an error code when usb_autoresume_device() is called.  This causes
libusb and/or lsusb to either hang or segfault.

If a USB device cannot suspend for some reason (e.g. a hub doesn't report
it has remote wakeup capabilities), we still want lsusb and other
userspace programs to work.  So return -EBUSY, which will fill people's
log files with failed tries, but will ensure userspace still works.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>