libata: export ata_get_cmd_descript()

The driver sata_dwc_460ex is using this symbol. To build it as a
module we have to have the symbol exported. This patch adds
EXPORT_SYMBOL_GPL() macro for that.

tj: Updated to use EXPORT_SYMBOL_GPL() instead of EXPORT_SYMBOL() as
    the only known user is an in-tree driver.  Suggested by Sergei.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>