Staging: hv: hv_mouse: use an anonymous union for struct mousevsc_prt_msg

Much nicer than having an ugly 'u.' in the structure usage, welcome to
the 2000's...

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