ata: fix sparse warning in sata_promise.c

drivers/ata/sata_promise.c:546:15: warning: symbol 'len' shadows an earlier one
drivers/ata/sata_promise.c:538:6: originally declared here

len is set again immediately after the loop, so this is safe.

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