Fix ACPI processor power block initialization

Properly clear the memory, and set "pr->flags.power" only if a C2 or
deeper state is valid (to make the code match both the comment and
previous behaviour).

This fixes a boot-time lockup reported by Maneesh Soni when using
"maxcpus=1".

Acked-by: Maneesh Soni <maneesh@in.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>