net: cdc_ncm: update datagram size after changing mtu

Current implementation updates the mtu size and notify cdc_ncm
device using USB_CDC_SET_MAX_DATAGRAM_SIZE request about datagram
size change instead of changing rx_urb_size.

Whenever mtu is being changed, datagram size should also be
updated. Also updating maxmtu formula so it takes max_datagram_size with
use of cdc_ncm_max_dgram_size() and not ctx.

Signed-off-by: Robert Dobrowolski <robert.dobrowolski@linux.intel.com>
Signed-off-by: Rafal Redzimski <rafal.f.redzimski@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>