MIPS: lantiq: fix early printk

The code was using a 32bit write operations in the early_printk code. This
resulted in 3 zero bytes also being written to the serial port. This patch
changes the memory access to 8bit.

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3721/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>