sata: fix reset_sata for dwc_ahsata

- fix crash when sata device is not initialized
- remove disable_sata_clock() since it is not clear which clock for which
  device should be disabled here
- call disable_sata_clock() for mx6 in preboot_os instead

Signed-off-by: Soeren Moch <smoch@web.de>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>
Tested-by: Nikita Kiryanov <nikita@compulab.co.il>