sata_dwc_460ex: enable COMPILE_TEST for the driver

To test how the driver could be compiled in the non-native environment let's
enable COMPILE_TEST for it. It would be useful for further work.

This patch enables COMPILE_TEST for the driver and fixes compilation errors on
at least x86 platforms.

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