class: rename "sem" to "class_sem" in internal class structure

This renames the struct class "sem" field to be "class_sem" 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>