net: cdc_ncm: refactoring cdc_ncm_setup

Rewriting the "set max datagram" part of dc_ncm_setup to
separate the selection and validatation of the size from
the code which optionally informs the device of this
value.  This ensures that we use the correct value
regardless of device support for the get and set commands.

Removing some of the many indent levels while doing this
to make the code more readable.

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