Fix the setting of the next adjacent fields in descriptors.

Following commit 5faf23ec the next_adj field of all descriptors is set
according to the index of the descriptor rather than its address which
causes issues when dma_alloc_coherent doesn't return an address which is
page aligned (which happens).
Moreover, in the case of a transfer which number of descriptors is
bigger than a full page, the next_adj field is set to the maximum (63)
for all descriptors untill the last page of descriptors where it starts
decreasing.
Last, even before this commit, the next_adj field inside a block of
adjacent descriptors is not decreasing untill coming near page end,
which is not compliant with what the documentation says :

"Every descriptor in the descriptor list must accurately describe the descriptor
or block ofdescriptors that follows. In a block of adjacent descriptors, the
Nxt_adj value decrements from the first descriptor to the second to last
descriptor which has a value of zero. Likewise, eachdescriptor in the block
points to the next descriptor in the block, except for the last descriptor
which might point to a new block or might terminate the list."

This commit aligns the blocks of adjacent descriptors to
XDMA_MAX_ADJ_BLOCK_SIZE and makes the next_adj field decrease inside
each block untill the second to last descriptor in the block or in the
full transfer. The size of the page being a multiple of the size of the
block (4096 = sizeof(xdma_desc) * 128 =
sizeof(xdma_desc) * 2 * XDMA_MAX_ADJ_BLOCK_SIZE