net: cdc_ncm: process chained NDPs

The NCM 1.0 spefication makes provisions for linking more than
one NDP into a single NTB. This is important for MBIM support,
where these NDPs might be of different types.

Following the chain of NDPs is also correct for NCM, and will
not change anything in the common case where there is only
one NDP

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