Staging: hv: hv_mouse: clean up camelcase when using struct hv_input_dev_info

I think there's a callback we can remove that uses this variable in the
future as well, but that's for another patch...

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>