CVE-2021-47469: spi: Fix deadlock when adding SPI controllers on SPI buses

In the Linux kernel, the following vulnerability has been resolved: spi: Fix deadlock when adding SPI controllers on SPI buses Currently we have a global spiaddlock which we take when adding new devices so that we can check that we're not trying to reuse a chip select that's already controlled. This means that if the SPI device is itself a SPI controller and triggers the instantiation of further SPI devices we trigger a deadlock as we try to register and instantiate those devices while in the process of doing so for the parent controller and hence already holding the global spiaddlock. Since we only care about concurrency within a single SPI bus move the lock to be per controller, avoiding the deadlock. This can be easily triggered in the case of spi-mux.