target/mips: Fix ADD.S FPU instruction

target/mips: Fix ADD.S FPU instruction

After merging latest QEMU upstream into our CHERI fork,
I noticed that some of the FPU tests in our MIPS baremetal
testsuite [*] started failing.
It turns out commit 1ace099f2a accidentally changed add.s
into a subtract.

[*] https://github.com/CTSRD-CHERI/cheritest

Fixes: 1ace099f2a ("target/mips: fpu: Demacro ADD.<D|S|PS>")
Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200703161515.25966-1-Alexander.Richardson@cl.cam.ac.uk>
Signed-off-by: Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>