net: cdc_ncm: refactor bind preparing for MBIM support

NCM and MBIM can share most of the bind function.  Split
out the shareable part and add MBIM functional descriptor
parsing.

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