MIPS: lantiq: fix cmdline parsing

The code tested if the KSEG1 mapped address of argv was != 0. We need to use
CPHYSADDR instead to make the conditional actually work.

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/3722/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>