binman: Call helper function binman_set_rom_offset() to fill offset

There is prepared function for filing rom_offset. That's why use it instead
of copying content of it.

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