sata_dwc.c: Fix GCC 4.6 build warnings

Fix:
sata_dwc.c: In function 'sata_dwc_softreset':
sata_dwc.c:444:5: warning: variable 'status' set but not used [-Wunused-but-set-variable]
sata_dwc.c:443:6: warning: variable 'serror' set but not used [-Wunused-but-set-variable]
sata_dwc.c: In function 'scan_sata':
sata_dwc.c:654:16: warning: variable 'lba_desc' set but not used [-Wunused-but-set-variable]
sata_dwc.c:538:16: warning: variable 'xfer_mask' set but not used [-Wunused-but-set-variable]
sata_dwc.c: In function 'ata_dev_read_id':
sata_dwc.c:747:14: warning: variable 'reason' set but not used [-Wunused-but-set-variable]
sata_dwc.c: In function 'ata_dev_read_sectors':
sata_dwc.c:1810:6: warning: variable 'rc' set but not used [-Wunused-but-set-variable]
sata_dwc.c: In function 'ata_dev_write_sectors':
sata_dwc.c:1994:6: warning: variable 'rc' set but not used [-Wunused-but-set-variable]

Signed-off-by: Stefan Roese <sr@denx.de>