MIPS: Make definitions of MIPSInst_FMA_{FUNC,FMTM} consistent with MIPS64 manual

The kernel definitions of MIPSInst_FMA_FUNC and MIPSInst_FMA_FFMT are not
consistent with MADD.fmt, NMADD.fmt and NMSUB.fmt in the MIPS64 manual [1],
the field func is bit 5..3 and fmt is bit 2..0, fix them. Otherwise there
exists error when add new instruction simulation.

[1] https://www.mips.com/?do-download=the-mips64-instruction-set-v6-06



Reported-by: Ming Wang <wangming01@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>