mips: use swiotlb_{alloc,free}

These already include the GFP_DMA/GFP_DMA32 usage, and will use CMA
memory if enabled, thus avoiding the GFP_NORETRY hack.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian König <christian.koenig@amd.com>