vmbus: don't return values for uninitalized channels

For unsupported device types, the vmbus channel ringbuffer is never
initialized, and therefore reading the sysfs files will return garbage
or cause a kernel OOPS.

Fixes: c2e5df61 ("vmbus: add per-channel sysfs info")

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: <stable@vger.kernel.org> # 4.15
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>