MIPS: microMIPS: Fix incorrect mask in insn_table_MM

It seems that this is a typo error and the proper bit masking is
"RT | RS" instead of "RS | RS".

This issue was detected with the help of Coccinelle.

Fixes: d6b3314b

 ("MIPS: uasm: Add lh uam instruction")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: <stable@vger.kernel.org> # 3.16+
Patchwork: https://patchwork.linux-mips.org/patch/17551/
Signed-off-by: James Hogan <jhogan@kernel.org>