ata: fix sparse warning in libata-core.c

rc is used to test the return value and possibly return an error.
No need to redeclare inside the loop.

drivers/ata/libata-core.c:7089:7: warning: symbol 'rc' shadows an earlier one
drivers/ata/libata-core.c:7030:9: originally declared here

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