spi: move unused device CS initialization to __spi_add_device()

Using spi_device::num_chipselect, initialize unused device CS as invalid
at registration time in __spi_add_device(), and drop it from the
different allocation paths.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Link: https://patch.msgid.link/20250915183725.219473-4-jonas.gorski@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>