storvsc: Fix typo in MODULE_PARM_DESC

The module_param is "storvsc_vcpus_per_sub_channel" so we need to use
that for MODULE_PARM_DESC() as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>