net: Convert fit verification to use fit_get_data_*

Several ethernet drivers load firmware from FIT images. Convert them to
use the fit_get_data helpers.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>