Cleanup DMA handling.

• Make isp_start() to set all the IOCB fields aside of S/G list, removing

extra information from isp_send_cmd(), now only doing S/G lists and sending.

• Turn DMA setup/free from being card and PCI-specific into OS-specific,

instead add new card-specific method for isp_send_cmd(). Previously this
function was a monster handling all the cards.

• Remove double error code translation.