ata: sata_rcar: really mask all interrupts on Gen2 and later

Since R-Car Gen2, a new bit has been introduced to the interrupt mask
register. Update the code to handle it properly as well.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tejun Heo <tj@kernel.org>