MIPS: Loongson2ef: Disable Loongson MMI instructions

It was missed when I was forking Loongson2ef from Loongson64 but
should be applied to Loongson2ef as march=loongson2f
will also enable Loongson MMI in GCC-9+.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Fixes: 71e2f4dd

 ("MIPS: Fork loongson2ef from loongson64")
Reported-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: stable@vger.kernel.org # v5.8+
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>