MIPS: LLVMLinux: Fix an 'inline asm input/output type mismatch' error.

Replace incorrect matching constraint that caused the error with an alternative
that still has the required constraints on the inline assembly.

This is the error message reported by clang:
arch/mips/include/asm/checksum.h:285:27: error: unsupported inline asm: input with type '__be32' (aka 'unsigned int') matching output with type 'unsigned short'
          "0" (htonl(len)), "1" (htonl(proto)), "r" (sum));
                                 ^~~~~~~~~~~~

The changed code can be compiled successfully by both gcc and clang.
Signed-off-by: NDaniel Sanders <daniel.sanders@imgtec.com>
Signed-off-by: NToma Tabacu <toma.tabacu@imgtec.com>
Suggested-by: NMaciej W. Rozycki <macro@linux-mips.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9313/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/checksum.h

@@ -218,6 +218,8 @@ static __inline__ __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
 					  __u32 len, unsigned short proto,
 					  __wsum sum)
 {
+	__wsum tmp;
+
 	__asm__(
 	"	.set	push		# csum_ipv6_magic\n"
 	"	.set	noreorder	\n"
@@ -270,9 +272,9 @@ static __inline__ __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
 
 	"	addu	%0, $1		# Add final carry\n"
 	"	.set	pop"
-	: "=r" (sum), "=r" (proto)
+	: "=&r" (sum), "=&r" (tmp)
 	: "r" (saddr), "r" (daddr),
-	  "0" (htonl(len)), "1" (htonl(proto)), "r" (sum));
+	  "0" (htonl(len)), "r" (htonl(proto)), "r" (sum));
 
 	return csum_fold(sum);
 }