scsi: Make private functions static

Several functions should be static because they are not exported to any
other file.
Warnings were reported by sparse C=1.

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