hv_netvsc: optimize initialization of RNDIS header

The memset of the whole maximum possible RNDIS header is unnecessary.
For the main part of the header use a structure assignment.

No need to memset the whole per packet info. Instead rely on caller to
set what it wants. Also get rid of cast to void and signed/unsigned
conversion. Now return pointer to per packet data (rather than the
header) which simplifies use by code setting up the packet data.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>