MIPS: ATH79: Avoid a kernel bug on AR913X

Wireless mac registration causes a BUG on AR913X SoCs due to
a missing 'else'.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3182/


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