Staging: hv: remove unneeded OnClose callback

This callback only calls one function, so just call the function
instead, no need for indirection at all.

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