staging: pi433: Fix rf69_set_tx_cfg() logic

Moved code to configure sync to where check enable_sync option before.
There is no need to check enable_sync twice. Configuring sync should be
executed immediately after enabling sync.

Signed-off-by: Sidong Yang <realwakka@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>