[IA64] fix nohalt boot option

this changeset broke the "nohalt" kernel boot option.
  8df5a500a3e97f7811cdce0f553ca1917ccd4220

default_idle() is looking at new variable can_do_pal_halt.  However,
that variable did not get cleared upon "nohalt" boot option.  Result
is that "nohalt" option is ignored until perfmon is exercised.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>