ethtool: cmis: validate fw->size against start_cmd_payload_size

ethtool: cmis: validate fw->size against start_cmd_payload_size

cmis_fw_update_start_download() copies start_cmd_payload_size bytes
from the firmware blob into the CDB LPL vendor_data[] payload without
validating that the FW has enough data.

Since the start_cmd_payload_size can only be ~120B an image too short
is most likely corrupted, so reject it.

Fixes: c4f78134d45c ("ethtool: cmis_fw_update: add a layer for supporting firmware update using CDB")
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Link: https://patch.msgid.link/20260522231312.1710836-10-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>