net: cdc_ncm: use IAD provided by the USB core

commit 9992c2e2 (net: cdc_ncm: workaround for missing CDC Union)
added code to lookup an IAD for the interface we are probing.
This is redundant.  The USB core has already done the lookup
and saved the result in the USB interface struct.  Use that
instead.

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