MIPS: math-emu: cp1emu: Fix typo when returning to register file

The commit 08a07904e182895e1205f399465a3d622c0115b8 (v3.16-rc1)
entitled "MIPS: math-emu: Remove most ifdefery":

switched from build time to runtime detection for the CPU ISA level.

However, along the way, a typo was introduced in the code path
to return the value to the register file. Previously, the
MIPSInst_FD macro was used but the above commit switched to
MIPSInst_RT leading to regressions.

Link: http://www.linux-mips.org/archives/linux-mips/2014-07/msg00484.html
Reported-by: Rob Kendrick <rob.kendrick@codethink.co.uk>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Rob Kendrick <rob.kendrick@codethink.co.uk>