net: cdc_ncm: log the length we warn about

Fix cut'n'paste typo.  Log the bogus length and not the
irrelevant signature.

Cc: Alexey Orishko <alexey.orishko@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>