hv_netvsc: Refactor assignments of struct netvsc_device_info

These assignments occur in multiple places. The patch refactor them
to a function for simplicity. It also puts the struct to heap area
for future expension.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
[sl: fix up subject line]
Signed-off-by: Sasha Levin <sashal@kernel.org>