Drivers: hv: vmbus: decrease num_sc on subchannel removal

It is unlikely that that host will ask us to close only one subchannel for a
device but let's be consistent. Do both num_sc++ and num_sc-- with
channel->lock to be on the safe side.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>