Fixes undefined variable in SPILock transfer by not using AsyncSPITransf...

...er methods

Hasn't been tested yet. 

SDCard was [broken by the last firmware update](https://forums.tessel.io/t/latest-firmware-update-breaks-microsd-example/609/4) and PR attempts to fix it. 
Length and chunk size should be
` computeBufferLength(rxbuf, txbuf)`
so `txbuf` is not required.