rfbridge: fix efm8bb1 provider send & receive

(possibly) noticed in the gitter
https://gitter.im/tinkerman-cat/espurna?at=61abba5a76e37917551e4a2f

refactoring causality, as there's no longer an explicit Serial.begin
for RFB_PROVIDER_EFM8BB1 in the hardware setup in the system.cpp
(also, from not including wip serial changes that did serial port setup :/)

code/espurna/rfbridge.cpp

@@ -1260,6 +1260,7 @@ void _rfbSettingsMigrate(int version) {
 
 void rfbSetup() {
 #if RFB_PROVIDER == RFB_PROVIDER_EFM8BB1
+    Serial.begin(SERIAL_BAUDRATE);
     _rfb_parser.reserve(RfbParser::MessageSizeBasic);
 #elif RFB_PROVIDER == RFB_PROVIDER_RCSWITCH