MIPS: ralink: fix USB frequency scaling

Commit 418d29c8 ("MIPS: ralink: Unify SoC id handling") was not fully
correct. The logic for the SoC check got inverted. We need to check if it
is not a MT76x8.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11992/


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