storvsc_on_channel_callback is a void function and the return
statement at the end is not useful.

Found with checkpatch.
Signed-off-by: Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>