spi_slave: fix crash issue caused by setting cs to -1
spi_slave: fix crash issue caused by setting cs to -1

Setting cs num to -1 means cs pin is not connected. But passing the value -1 to the
spicommon_cs_initialize() will lead to program crash caused by "array index out
of bounds". We expect the driver to execute well but the driver should not be in
charge of users' behaviour, so this commit prevents the crash.

https://github.com/espressif/esp-idf/issues/5784

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf