spi: dw: Bail out early on unsupported target mode

Currently, the DesignWare SPI controller driver supports only host mode.
However, spi2 on the Kendryte K210 SoC supports only target mode,
triggering an error message on e.g. SiPEED MAiXBiT since commit
98d75b9e ("spi: dw: Drop default number of CS setting"):

    dw_spi_mmio 50240000.spi: error -22: problem registering spi host
    dw_spi_mmio 50240000.spi: probe with driver dw_spi_mmio failed with error -22

As spi2 rightfully has no "num-cs" property, num_chipselect is now zero,
causing spi_alloc_host() to fail to register the controller.  Before,
the driver silently registered an SPI host controller with 4 chip
selects.

Reject target mode early on and warn the user, getting rid of the
error message.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/7ae28d83bff7351f34782658ae1bb69cc731693e.1715163113.git.geert+renesas@glider.be


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