[POWERPC] of_serial: Ignore unknown serial port

Current of_serial driver assumes unknown serial port to be 8250.  But
this behavior may cause fatal problems when the serial port is not
8250.  This corrects probe routine to ignore unknown serial port.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>