libata: make atapi_request_sense() use sg

atapi_request_sense() is now the only left user of ata_sg_init_one().
Convert it to use sg interface.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Rusty Russel <rusty@rustcorp.com.au>
Signed-off-by: Jeff Garzik <jeff@garzik.org>