MIPS: mm: remove duplicate "const" qualifier on insn_table

Fixes the following gcc 7.x build error:

arch/mips/mm/uasm-mips.c:51:26: error: duplicate ‘const’ declaration specifier [-Werror=duplicate-decl-specifier]
 static const struct insn const insn_table[insn_invalid] = {

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Fixes: ce807d5f

 ("MIPS: Optimize uasm insn lookup.")
Cc: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16926/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>