net: cdc_ncm: fix SET_MAX_DATAGRAM_SIZE

We need to inform the device about the *new* value, not the
old one.

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