[PATCH] powerpc: fix compile warning in udbg_init_maple_realmode

arch/powerpc/kernel/udbg_16550.c: In function `udbg_init_maple_realmode':
arch/powerpc/kernel/udbg_16550.c:162: warning: assignment from incompatible pointer type

Signed-off-by: Olaf Hering <olh@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>