skd: Simplify the code for allocating DMA message buffers

dma_alloc_coherent() guarantees alignment on a page boundary so
no explicit alignment is needed to align on a 64 byte boundary.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>