Add missing variable initialisation to make SPI work for multiple CS
Ref https://github.com/espressif/esp-idf/pull/277
From PR: "I am not especially familiar with this platform or with esp-idf -- so review with care.
The spi-master driver would not work for me when the number of devices on a bus exceeded 1 (caused device hang). I believe this is because cur_cs is not properly set (anywhere), and yet is used. This is likely just not noticed because 0 is presumably the correct value when there's only 1 device (and I'm guessing multi-device setup is not tested as thoroughly).
Simply setting cur_cs to the correct slot value resolves the problem and multiple devices on a single bus behave correctly in my test environment."
Looks like a small oversight on my side.
See merge request !468
a4c76671c2