MIPS: XPA: Allow use of $0 (zero) to MTHC0

Tweak __writex_32bit_c0_register() to allow the compiler to use $0 (the
zero register) as an input to the mthc0 instruction.
Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17774/

arch/mips/include/asm/mipsregs.h

@@ -1537,10 +1537,10 @@ do {									\
 	"	.set	push					\n"	\
 	"	.set	mips32r2				\n"	\
 	_ASM_SET_XPA							\
-	"	mthc0	%0, $%1					\n"	\
+	"	mthc0	%z0, $%1				\n"	\
 	"	.set	pop					\n"	\
 	:								\
-	: "r" (value), "i" (register));					\
+	: "Jr" (value), "i" (register));				\
 } while (0)
 
 #define read_c0_index()		__read_32bit_c0_register($0, 0)