[PATCH] Fix compiler warning in driver core for CONFIG_HOTPLUG=N

FYI, while running a build test, I found:

drivers/base/bus.c:166: warning: `driver_attr_unbind' defined but not used
drivers/base/bus.c:194: warning: `driver_attr_bind' defined but not used

Looks like these two attributes and supporting functions want to be
#ifdef HOTPLUG'd

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>