ata: fix sparse warning in ata_piix.c

drivers/ata/ata_piix.c:1655:8: warning: symbol 'rc' shadows an earlier one
drivers/ata/ata_piix.c:1616:6: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>