ahci: Support splitting of read transactions into multiple chunks

With an Intel AHCI controller, the driver does not operate properly
if the requested amount of blocks to read exceeds 255.

It is probably possible to specify 0 as the block count and the driver
will read 256 blocks, but it was decided to limit the number of blocks
read at once to 128 (it should be a power of 2 for the optimal
performance of solid state drives).

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>