MIPS: Loongson-3: Fix fp register access if MSA enabled

If MSA is enabled, FPU_REG_WIDTH is 128 rather than 64, then get_fpr64()
/set_fpr64() in the original unaligned instruction emulation code access
the wrong fp registers. This is because the current code doesn't specify
the correct index field, so fix it.

Fixes: f83e4f98 ("MIPS: Loongson-3: Add some unaligned instructions emulation")
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Pei Huang <huangpei@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>