Input: sparcspkr - use platform_register/unregister_drivers()

These new helpers simplify implementing multi-driver modules and
properly handle failure to register one driver by unregistering all
previously registered drivers.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>