Implement SPISlave.end()

[svofski]

My use case is simple: I have a system where devices change roles. ESP begins being a master and configures an FPGA, which then takes over as a master, and ESP becomes a slave. Later on, a firmware update may require this process to repeat, etc. SPI bus role reversal is not very typical, but completely possible, especially with pin shortage or tightness of PCB estate.

A PR for this request: #4248

[devyte]

Per @Tech-TX:
Closing due to #4248 already merged.