MIPS: ralink: Fix invalid tick count

The current code adds the delta twice, which is obviously wrong.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11443/


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