Staging: hv: hv_mouse: change camelcase for struct synthhid_msg

Turns out no one references the data field of this structure, so I
wonder if it's really even needed at all.  All this is used for is the
type of the message here, so this structure might be able to be dropped
entirely in the future.

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>