MIPS: R2-on-R6 MULTU/MADDU/MSUBU emulation bugfix

MIPS: R2-on-R6 MULTU/MADDU/MSUBU emulation bugfix

MIPS instructions MULTU, MADDU and MSUBU emulation requires registers HI/LO
to be converted to signed 32bits before 64bit sign extension on MIPS64.

Bug was found on running MIPS32 R2 test application on MIPS64 R6 kernel.

Fixes: b0a668fb2038 ("MIPS: kernel: mips-r2-to-r6-emul: Add R2 emulator for MIPS R6")
Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Reported-by: Nikola.Veljkovic@imgtec.com
Cc: paul.burton@imgtec.com
Cc: yamada.masahiro@socionext.com
Cc: akpm@linux-foundation.org
Cc: andrea.gelmini@gelma.net
Cc: macro@imgtec.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14043/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>