USB: serial: mxuport: validate firmware header size

USB: serial: mxuport: validate firmware header size

mxuport_probe() reads version bytes at fixed offsets after
request_firmware() succeeds. Firmware loading success does not prove that
the blob reaches the highest version offset.

Reject short firmware images before reading the version bytes. This is
source-level parser hardening; no affected device or crash was observed.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Fixes: ee467a1f2066 ("USB: serial: add Moxa UPORT 12XX/14XX/16XX driver")
Signed-off-by: Johan Hovold <johan@kernel.org>