[PATCH] acpi_shutdown: Only prepare for power off on power_off

When acpi_sleep_prepare was moved into a shutdown method we
started calling it for all shutdowns.

It appears this triggers some systems to power off on reboot.

Avoid this by only calling acpi_sleep_prepare if we are going to power
off the system.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>