MIPS: Correct `mmiowb' barrier for `wbflush' platforms

Redefine `mmiowb' in terms of `iobarrier_w' so that it works correctly
for MIPS I platforms, which have no SYNC machine instruction and use a
call to `wbflush' instead.

This doesn't change the semantics for CONFIG_CPU_CAVIUM_OCTEON, because
`iobarrier_w' expands to `wmb', which is ultimately the same as the
current arrangement.  For MIPS I platforms this not only makes any code
that would happen to use `mmiowb' build and run, but it actually
enforces the ordering required as well, as `iobarrier_w' has it already
covered with the use of `wmb'.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20863/
Cc: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/io.h

@@ -97,6 +97,9 @@ static inline void set_io_port_base(unsigned long base)
 #define iobarrier_w() wmb()
 #define iobarrier_sync() iob()
 
+/* Some callers use this older API instead.  */
+#define mmiowb() iobarrier_w()
+
 /*
  *     virt_to_phys    -       map virtual addresses to physical
  *     @address: address to remap
@@ -545,14 +548,6 @@ BUILDSTRING(l, u32)
 BUILDSTRING(q, u64)
 #endif
 
-
-#ifdef CONFIG_CPU_CAVIUM_OCTEON
-#define mmiowb() wmb()
-#else
-/* Depends on MIPS II instruction set */
-#define mmiowb() asm volatile ("sync" ::: "memory")
-#endif
-
 static inline void memset_io(volatile void __iomem *addr, unsigned char val, int count)
 {
 	memset((void __force *) addr, val, count);