mtip32xx: fully switch to the generic DMA API

The mtip32xx used an odd mix of the old PCI and the generic DMA API,
so switch it over to the generic API entirely.

Note that this also removes a weird fallback to just a 32-bit coherent
dma mask if the 64-bit dma mask doesn't work, as that can't even happen.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>