MIPS: Loongson64: Fix empty-body warning in dma_alloc

MIPS: Loongson64: Fix empty-body warning in dma_alloc

A new gcc warning shows up for this old code with gcc-6:

arch/mips/loongson64/common/dma-swiotlb.c: In function 'loongson_dma_alloc_coherent':
arch/mips/loongson64/common/dma-swiotlb.c:35:2: error: suggest braces around empty body in an 'else' statement [-Werror=empty-body]

The code can be easily restructured to look more readable
and avoid the warning at the same time.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15047/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>