hv_netvsc: remove vmbus_are_subchannels_present() in rndis_filter_device_add()

The vmbus_are_subchannels_present() also involves opening the channels, which
may be too early at this point. Checking for subchannels is not necessary here.
So this patch removes it. Subchannels will be opened when offer messages arrive.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>