Staging: hv: make *context a real pointer in struct hv_device

This is really the struct vmbus_channel for the device, not a void
pointer, so use the real structure to ensure type safety everywhere.

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