otPlatSpiSlaveTransactionCompleteCallback

Indicates that a SPI transaction has completed with the given length. The data written to the slave has been written to the pointer indicated by the `aInputBuf` argument to the previous call to `otPlatSpiSlavePrepareTransaction()`. Once this function is called, `otPlatSpiSlavePrepareTransaction()` is invalid and must be called again for the next transaction to be valid. Note that this function is always called at the end of a transaction, even if `otPlatSpiSlavePrepareTransaction()` has not yet been called. In such cases, `aOutputBufLen` and `aInputBufLen` will be zero. This callback can be called from ISR context. The return value from this function indicates if any further processing is required. If `TRUE` is returned the platform spi-slave driver implementation must invoke the transaction process callback (`aProcessCallback` set in `otPlatSpiSlaveEnable()`) which unlike this callback must be called from the same OS context that any other OpenThread API/callback is called.