MIPS: ralink: Fix clearing the illegal access interrupt

Due to a typo the illegal access interrupt is never cleared in by
the interupt handler, causing an effective deadlock on the first
illegal access.

This was broken since the code was introduced in 5433acd8 ("MIPS:
ralink: add illegal access driver"), but only exposed when the Kconfig
symbol was added, thus enabling the code.

Cc: <stable@vger.kernel.org>  [3.18+]
Fixes: a7b7aad3 ("MIPS: ralink: add missing symbol for RALINK_ILL_ACC")
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: John Crispin <blogic@openwrt.org>
Patchwork: https://patchwork.linux-mips.org/patch/10172/


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