disk: sdhc: spi: leave SPI CS GPIO handling to the SPI controller driver

Leave the SPI CS GPIO handling to the SPI controller driver and make SPI
CS GPIO optional for SDHC disk access via SPI.

The SPI SDHC implementation relies on SPI_HOLD_ON_CS so we need to
explicitly release the SPI controller driver after ending transmissions
to force the CS line to be de-asserted.

Fixes #27444.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>