net: cdc_ncm: split out rx_max/tx_max update of setup

Split out the part of setup dealing with updating the rx_max
and tx_max buffer sizes so that this code can be reused for
dynamically updating the limits.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>