MIPS: MSA: Fix big-endian FPR_IDX implementation

The maximum word size is 64-bits since MSA state is saved using st.d
which stores two 64-bit words, therefore reimplement FPR_IDX using xor,
and only within each 64-bit word.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9169/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/processor.h

@@ -105,7 +105,7 @@ union fpureg {
 #ifdef CONFIG_CPU_LITTLE_ENDIAN
 # define FPR_IDX(width, idx)	(idx)
 #else
-# define FPR_IDX(width, idx)	((FPU_REG_WIDTH / (width)) - 1 - (idx))
+# define FPR_IDX(width, idx)	((idx) ^ ((64 / (width)) - 1))
 #endif
 
 #define BUILD_FPR_ACCESS(width) \