acpi: fix oops in acpi_system_wakeup_device_seq_show

Commit 0794469d: ("ACPI: struct device -
replace bus_id with dev_name(), dev_set_name()") introduced a bug by
testing 'dev_name(ldev)' instead of 'ldev->bus' for NULL when printing
out the bus information.

So if ldev->bus was NULL, we'd oops.

Reported-and-tested-by: Bruno Prmont <bonbons@linux-vserver.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Len Brown <len.brown@intel.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>