spl: spl_legacy: fix invalid offset in SPL_COPY_PAYLOAD_ONLY

spl: spl_legacy: fix invalid offset in SPL_COPY_PAYLOAD_ONLY

This fixes the header offset calculation.

This issue was found on uniphier v7 SoCs with SPL.

Fixes: f6729aceea ("spl: spl_legacy: Fix NAND boot on OMAP3 BeagleBoard")
Signed-off-by: Dai Okamura <okamura.dai@socionext.com>
Reviewed-By: Michael Trimarchi <michael@amarulasolutions.com>
Acked-by: Michael Trimarchi <michael@amarulasolutions.com>
Link: https://lore.kernel.org/all/20221209114021.3074978-1-okamura.dai@socionext.com
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>