x86/PCI: Describe @reg for type1_access_ok()

Describe missed parameter in documentation of type1_access_ok().
Otherwise "make W=1 arch/x86/pci/" produces the following warning:
  CHECK   arch/x86/pci/intel_mid_pci.c
  CC      arch/x86/pci/intel_mid_pci.o
  arch/x86/pci/intel_mid_pci.c:152: warning: Function parameter or member 'reg' not described in 'type1_access_ok'

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>