class: rename "interfaces" to "class_interfaces" in internal class structure

This renames the struct class "interfaces" field to be
"class_interfaces" to make things easier when struct bus_type and struct
class merge in the future.  It also makes grepping for fields easier as
well.

Based on an idea from Kay.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>