scsi: Change scsi_scan() to be able to return value

With DM_SCSI this function will return more than one return value to
cover errors.

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/scsi.h

@@ -173,7 +173,7 @@ void scsi_low_level_init(int busdevfunc);
  * functions residing inside cmd_scsi.c
  */
 void scsi_init(void);
-void scsi_scan(int mode);
+int scsi_scan(int mode);
 
 #define SCSI_IDENTIFY					0xC0  /* not used */