efi_loader: capsule: enforce guid check in api and capsule_on_disk

While UPDATE_CAPSULE api is not fully implemented, this interface and
capsule-on-disk feature should behave in the same way, especially in
handling an empty capsule for fwu multibank, for future enhancement.

So move the guid check into efi_capsule_update_firmware().

Fixed: commit a6aafce4 ("efi_loader: use efi_update_capsule_firmware() for capsule on disk")
Reported-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reported-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>