HPET: fix ACPI memory range length handling

ACPI address space descriptors contain _MIN, _MAX, and _LEN.  _MIN and _MAX
are the bounds within which the region can be moved (this is clarified in
Table 6-38 of the ACPI 3.0 spec).  We should use _LEN to determine the size
of the region, not _MAX - _MIN + 1.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>