cdc_ncm: fix MTU and max_datagram_size handling

Changes/fixes:
- inform device if max_datagram_size was changed by host
- max_datagram_size can't be bigger MTU in ETH func descr
- fix constants definitions to enable running CAIF service over NCM

Tested on Intel/ARM.

Reviewed-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Tested-by: Dmitry Tarnyagin <Dmitry.Tarnyagin@stericsson.com>
Signed-off-by: Alexey Orishko <alexey.orishko@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>