PCI: rpaphp: Document is_php_dn()

Fix up the documentation: the rpaphp_add_slot() does not actually
handle embedded slots: in fact, it ignores them. Fix the flow of
control in the routine that checks for embedded slots.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Cc: John Rose <johnrose@austin.ibm.com>
Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>