[IA64] Fix pcibios_setup

pcibios_setup() should return NULL if it handled a parameter.  Since ia64
handles no parameters, it should return the string that was passed in,
not NULL.  This brings ia64 into line with all other architectures that
handle no parameters.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Tony Luck <tony.luck@intel.com>