[POWERPC] Use alloc_maybe_bootmem() in pcibios_alloc_controller

Use alloc_maybe_bootmem() which wraps the if (mem_init_done)
malloc clause.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>