efi: Drop error return in efi_carve_out_dt_rsv()

This function currently returns an error code, but never uses it. There is
no function comment so it is not obvious why. Presuambly the error is not
important.

Update the function to explain its purpose and why it ignores the error.
Drop the useful error return value.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>