MIPS: R46000: Fix Micro-assembler field overflow for R4600 V2

Fix uasm warning, which triggered because of workaround for R4600 V2 CPUs.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6716/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>