imx_sdp: fix fetch_data
imx_sdp: fix fetch_data

Make sure cnt returned does not overlap header.
For file load, there is no header to return.

Signed-off-by: Troy Kisky <debfc29e3a9e989b865b88f1f207951588bac6b3@boundarydevices.com>

boundarydevices/imx_usb_loader,bwalle/imx_usb_loader