Staging: hv: hv_mouse: use proper input define for bus type

The code was so close, the bus type was in a comment, so go all the way
and actually use the define here.  It's as if the original author was so
afraid of license issues if they referenced a define in the processed
code but they felt safe to keep it in a comment.  Chicken.

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>