MIPS: ralink: Fix incorrect assignment on ralink_soc

ralink_soc sould be assigned to RT3883_SOC, replace incorrect
comparision with assignment.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Fixes: 418d29c8 ("MIPS: ralink: Unify SoC id handling")
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14903/


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