isci: Make the driver copy data directly from and to sg for PIO

We can copy the data directly to and from sg for SATA PIO read operations.
There is no reason to involve the hardware SGL. In the process we also need
to kmap the sg because we don't know where that can come from.

We also do to not call phys_to_virt(). The driver already has the information.
We can just calculcate the appropriate offets.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>