Implement per-dev xfermask.  libata used to determine xfermask
per-port - the fastest mode of the slowest device on the port.  This
patch enables per-dev xfermask.

Original patch is from Alan Cox <alan@redhat.com>.  The following
changes are made by me.

* simplex warning message is added
* remove disabled device handling code which is never invoked
  (originally for choosing port-wide lowest PIO mode)

Cc: Alan Cox <alan@redhat.com>
Signed-off-by: Tejun Heo <htejun@gmail.com>