net: cdc_ncm: only the control intf can be probed

The probed interface must be the master/control interface of the
function.  Make this explicit and simplify redundant tests.

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