ata/sata_fsl: Remove unnecessary SCR cases

SCRs in the driver map to the standard values found in include/linux/ata.h,
so no need for individual scr_read/scr_write case statements duplicating
the natural value.

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>