udbg_read would lock up if ppc_md.udbg_putc didnt exist.  Just return 0 and
dont lock up instead.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>