USB: serial: io_ti: reject oversized boot-mode firmware

USB: serial: io_ti: reject oversized boot-mode firmware

do_boot_mode() copies the firmware payload, excluding its four-byte prefix,
into a fixed 15.5 KiB staging buffer. check_fw_sanity() already proves that
the image contains its seven-byte header and validates the declared image
length and checksum, but it does not impose this boot-mode destination
limit.

Reject images whose payload does not fit before allocating and filling the
staging buffer.

Fixes: d12b219a228e ("edgeport-ti: use request_firmware()")
Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Signed-off-by: Johan Hovold <johan@kernel.org>