pciehp: don't enable slot unless forced

This fixes a 2.6.25 regression reported by Alex Chiang.

Invoke pciehp_enable_slot() at startup only when pciehp_force=1.
Some HP equipment apparently cannot cope with it otherwise.

This restores the (previously working) 2.6.24 behaviour here,
while allowing machines that need a kick to use pciehp_force=1.

This was the original design back in October 2007,
but Kristen suggested we try without it first:

   Kristen Carlson Accardi wrote:
   >I think it would be ok to try allowing the slot to be enabled when not
   >using pciehp_force mode.  We can wrap it later if it proves to break things

This ended up breaking one of Alex's setups,
so it's time to put the wrapper back in now.

Signed-off-by: Mark Lord <mlord@pobox.com>
Acked-by: Alex Chiang <achiang@hp.com>
Acked-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>