Hadware SPI Slave Select (SS) setup and hold bit states in SPI1U not maintained

I had some issues with lost SPI data in overlap mode. Tracked this down to the Slave Select (SS) setup and hold bits getting cleared by different SPI calls.

The bits are set [here](https://github.com/esp8266/Arduino/blob/master/libraries/SPI/SPI.cpp#L127) by the call [here](https://github.com/esp8266/Arduino/blob/master/libraries/SPI/SPI.cpp#L72).

But then immediately afterwards cleared [here](https://github.com/esp8266/Arduino/blob/master/libraries/SPI/SPI.cpp#L84). These bits are also cleared elsewhere in the SPI functions, [here ](https://github.com/esp8266/Arduino/blob/master/libraries/SPI/SPI.cpp#L472)and [here](https://github.com/esp8266/Arduino/blob/master/libraries/SPI/SPI.cpp#L496) and [here](https://github.com/esp8266/Arduino/blob/master/libraries/SPI/SPI.cpp#L508).

Edit (devyte):
Ref commit [here](https://github.com/esp8266/Arduino/commit/8c01516f8ab0931d93ed8a4a97957bfe3ea4a7a6).