Staging: hv: Get rid of the volatile qualifier for a variable in ring_buffer.c

Staging: hv: Get rid of the volatile qualifier for a variable in ring_buffer.c

Access to this variable is already serialized via a spin lock. Get
rid of the unnecessary volatile qualifier.

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: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>