[POWERPC] of_serial: add port type checking

Currently, of_serial driver supports only 8250 compatible ports, but
prepares that others can easily be added.  Yet unregister routine
assumes port is 8250.  This corrects unregister routine to check
port type.

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>