CDC-NCM: handle incomplete transfer of MTU

[ Upstream commit 332f989a3b0041b810836c5c3747e59aad7e9d0b ]

A malicious device may give half an answer when asked
for its MTU. The driver will proceed after this with
a garbage MTU. Anything but a complete answer must be treated
as an error.

V2: used sizeof as request by Alexander

Reported-and-tested-by:  <syzbot+0631d878823ce2411636@syzkaller.appspotmail.com>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>