MIPS: cavium_octeon: Fix syncw generation.

The Cavium Octeon CPU uses a special sync instruction for implementing
wmb, and due to a CPU bug, the instruction must appear twice. A macro
had been defined to hide this:

 #define __SYNC_rpt(type)     (1 + (type == __SYNC_wmb))

which was intended to evaluate to 2 for __SYNC_wmb, and 1 for any other
type of sync. However, this expression is evaluated by the assembler,
and not the compiler, and the result of '==' in the assembler is 0 or
-1, not 0 or 1 as it is in C. The net result was wmb() producing no code
at all. The simple fix in this patch is to change the '+' to '-'.

Fixes: bf929272

 ("MIPS: barrier: Add __SYNC() infrastructure")
Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org