sata_rcar: fix compilation warning in sata_rcar_thaw()

When compiling the driver with gcc 4.8, it gives the following warning:

 drivers/ata/sata_rcar.c: In function `sata_rcar_thaw':
 drivers/ata/sata_rcar.c:183:2: warning: large integer implicitly truncated to unsigned type [-Woverflow]

Fix the warning by explicit cast of the 'unsigned long' value to 'u32'.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tejun Heo <tj@kernel.org>