net: cdc_ncm: factor out one-time device initialization

Split the parts of setup dealing with device initialization from
parts just setting defaults for attributes which might be
changed after initialization.

Some commands of the device initialization are only allowed when
the data interface is in its disabled altsetting, so we must
separate them out of we are to allow rerunning parts of setup.
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>