sbus: convert drivers/sbus/char/* to use module_platform_driver()

This patch converts the drivers in drivers/sbus/char/* to use the
module_platform_driver() macro which makes the code smaller and a bit
simpler.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>