LoongArch: Fix wrong fpu version

According to the configuration information accessible by the CPUCFG
instruction in LoongArch Reference Manual [1], FP_ver is stored in
bit [5: 3] of CPUCFG2, the current code to get fpu version is wrong,
use CPUCFG2_FPVERS to fix it.

[1] https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

Fixes: 628c3bb4

 ("LoongArch: Add boot and setup routines")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>