ata: ahci_st: fixup layering violations / drvdata errors

Brian noticed while working on another SATA driver that uses libahci_platform,
an error in this driver; it tries to the the driver data for its
device, while libata also thinks it can set the driver data. See:

  ahci_platform_init_host()
  -> ata_host_alloc_pinfo()
     -> ata_host_alloc()
        -> dev_set_drvdata()

So instead of sticking the IP-specific platform data into drvdata, let's
use the plat_data variable that is reserved for this use.

Addtionally plat_data isn't set until ahci_platform_init_host() has been
called further down in probe(). So re-work the st_ahci_probe_resets and
st_ahci_deassert_resets functions to take ahci_host_priv *hpriv as a
parameter.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Suggested-by: Brian Norris <computersforpeace@gmail.com>
Cc: Srinivas Kandagatla <srinivas.kandagatla@gmail.com>
Cc: Maxime Coquelin <maxime.coquelin@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Tejun Heo <tj@kernel.org>