Fix the following gcc 7.x build error on microMIPS builds:

arch/mips/mm/uasm-micromips.c:43:26: error: duplicate ‘const’ declaration specifier [-Werror=duplicate-decl-specifier]
 static const struct insn const insn_table_MM[insn_invalid] = {
                          ^~~~~

The same issue has already been fixed in uasm-mips by commit
00e06297 ("MIPS: mm: remove duplicate "const" qualifier on
insn_table").
Signed-off-by: James Hogan <jhogan@kernel.org>
Fixes: ce807d5f ("MIPS: Optimize uasm insn lookup.")
Cc: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17889/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>