spl: spl_imx_container: Delete uninitialized variable

spl: spl_imx_container: Delete uninitialized variable

The 'overhead' variable is uninitialized and actually shall not be used.
Delete it to fix coverity CID 37041718 - Uninitialized scalar variable.

Fixes: 73c40fcb7367 ("spl: Refactor spl_load_info->read to use units of bytes")
Reported-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Andrew Goodbody <andrew.goodbody@linaro.org>