aspeed: check for negative values returned by blk_getlength()

write_boot_rom() does not check for negative values. This is more a
problem for coverity than the actual code as the size of the flash
device is checked when the m25p80 object is created. If there is
anything wrong with the backing file, we should not even reach that
path.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 1486648058-520-2-git-send-email-clg@kaod.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
