libata-scsi: clean up inquiry / mode sense related functions

* make ata_scsiop_*() static
* make ata_scsi_set_sense() static and move it above its users
* make ata_scsi_rbuf_fill() static
* kill unused ata_scsi_badcmd()

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>