sata_dwc_460ex: convert to devm_kzalloc in ->probe()

The patch converts ->probe() to use devm_kzalloc that simplifies error path.
Note that ata_host_alloc_pinfo() has been using device resources already.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>