advansys: Use dma_pool for sg elements

The sg elements should be allocated from a dma pool.
And rename the structure to 'adv_sg_block' as they
are only used by the wide board.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>