Fix Serial::writeable() and txIrq for nRF15822

With the old logic if Serial::writeable() was called before and/or used to block calls to Serial::putc() it would never write due to EVENT_TXDRDY trailing a write to TXD. Add a dummy write to TXD before pins are connected to the peripheral. This primes the EVENT_TXDRDY to lead future writes rather than trail. Since STOPTX isn't used this seems safe.