staging: pi433: Split rf69_set_sync_enabled into two functions

Splits rf69_set_sync_enabled(dev, enabled) into
rf69_enable_sync(dev) and rf69_disable_sync(dev).

Signed-off-by: Simon Sandström <simon@nikanor.nu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>