efi_loader: image_loader: fix a Coverity check against array access

efi_loader: image_loader: fix a Coverity check against array access

Coverity detected:
  Using "&opt->CheckSum" as an array.  This might corrupt or misinterpret
  adjacent memory locations.

The code should work as far as a structure, IMAGE_OPTIONAL_HEADER(64) is
packed, but modify it in more logical form. Subsystem is a member next to
CheckSum.

Reported-by: Coverity (CID 300339)
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>