USB: Let usb_sg_init to set transfer_buffer more often

			io->urbs[i]->context = io;

			/*

			 * Some systems need to revert to PIO when DMA is

			 * temporarily unavailable.  For their sakes, both

			 * transfer_buffer and transfer_dma are set when

			 * possible.  However this can only work on systems

			 * without:

			 *

			 *  - HIGHMEM, since DMA buffers located in high memory

			 *    are not directly addressable by the CPU for PIO;

			 *

			 *  - IOMMU, since dma_map_sg() is allowed to use an

			 *    IOMMU to make virtually discontiguous buffers be

			 *    "dma-contiguous" so that PIO and DMA need diferent

			 *    numbers of URBs.

			 *

			 * So when HIGHMEM or IOMMU are in use, transfer_buffer

			 * is NULL to prevent stale pointers and to help spot

			 * bugs.

			 * Some systems need to revert to PIO when DMA is temporarily

			 * unavailable.  For their sakes, both transfer_buffer and

			 * transfer_dma are set when possible.

			 *

			 * Note that if IOMMU coalescing occurred, we cannot

			 * trust sg_page anymore, so check if S/G list shrunk.

			 */

			if (io->nents == io->entries && !PageHighMem(sg_page(sg)))

				io->urbs[i]->transfer_buffer = sg_virt(sg);

			else

				io->urbs[i]->transfer_buffer = NULL;

			if (dma) {

				io->urbs[i]->transfer_dma = sg_dma_address(sg);

				len = sg_dma_len(sg);

#if defined(CONFIG_HIGHMEM) || defined(CONFIG_GART_IOMMU)

				io->urbs[i]->transfer_buffer = NULL;

#else

				io->urbs[i]->transfer_buffer = sg_virt(sg);

#endif

			} else {

				/* hc may use _only_ transfer_buffer */

				io->urbs[i]->transfer_buffer = sg_virt(sg);

				len = sg->length;

			}