MIPS: RB532: Fix reregistering of serial console

MIPS: RB532: Fix reregistering of serial console

Runtime tested on Mikrotik RB532 board.
Thanks goes to Geert Uytterhoeven for the explanation of the problem.

"I'm afraid this is not gonna help. When the port is unregistered,
its type will be reset to PORT_UNKNOWN.
So before registering it again, its type must be set again the actual
serial driver, cfr. the change to of_serial.c."

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Reviewed-by: Florian Fainelli <florian@openwrt.org>
Cc: Linux MIPS Mailing List <linux-mips@linux-mips.org>
Cc: geert@linux-m68k.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/7241/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>