USB: mention URB_FREE_BUFFER in usb_free_urb documentation

The usb_free_urb comment says that the transfer buffer will not be
freed, but this is not the case when URB_FREE_BUFFER is set.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>