sparc: piggyback: handle invalid image

With an old elftoaout, the generation of tftpboot.img fail with "lseek:
invalid argument".
This is due to offset being negative.

Instead of printing this error message, let's print a better one.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>