MIPS: mm: Fix definition of R6 cache instruction

Commit a168b8f1 ("MIPS: mm: Add MIPS R6 instruction encodings") added
an incorrect definition of the redefined MIPSr6 cache instruction.

Executing any kernel code including this instuction results in a
reserved instruction exception and kernel panic.

Fix the instruction definition.

Fixes: a168b8f1
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: <stable@vger.kernel.org> # 4.x-
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13663/


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