Staging: hv: vmbus: Fix a memory barrier call in vmbus_drv.c

Use the correct memory barrier call in vmbus_drv.c

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