Staging: hv: vmbus: Check before invoking the channel callback

When we close a channel, we set the corresponding callback function to NULL.
Check before invoking the channel callback.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>