MIPS: Octeon: Avoid empty-body warning

gcc-6 reports a harmless build warning:

arch/mips/cavium-octeon/dma-octeon.c: In function 'octeon_dma_alloc_coherent':
arch/mips/cavium-octeon/dma-octeon.c:179:3: error: suggest braces around empty body in an 'else' statement [-Werror=empty-body]

We can fix this by rearranging the code slightly using the
IS_ENABLED() macro.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15048


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>