HID: vivaldi: Minor cleanups

Perform some minor cleanups on this driver. Include header files for
struct definitions that are used, drop a forward declaration that isn't
useful, and mark a sysfs attribute static as it isn't used outside this
file.

Cc: Sean O'Brien <seobrien@chromium.org>
Cc: Ting Shen <phoenixshen@chromium.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>