spi: cadence-qspi: set maximum chip-select to 4

Change the maximum chip-select count in cadence-qspi to 4 instead of 16.
The value gets used as default ->num_chipselect when the num-cs DT
property isn't received from devicetree. It also determines the
cqspi->f_pdata array size.

Hardware only supports values up to 4; see cqspi_chipselect() that sets
CS using a one-bit-per-CS 4-bit register field.

Add a static_assert() call as a defensive measure to ensure we stay
under the SPI subsystem limit. It got set to 4 when introduced in
4d8ff6b099 ("spi: Add multi-cs memories support in SPI core") and
later increased to 16 in 2f8c7c3715 ("spi: Raise limit on number of
chip selects").

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://msgid.link/r/20240209-cdns-qspi-cs-v1-2-a4f9dfed9ab4@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-cadence-quadspi.c

@@ -31,7 +31,9 @@
 #include <linux/timer.h>
 
 #define CQSPI_NAME			"cadence-qspi"
-#define CQSPI_MAX_CHIPSELECT		16
+#define CQSPI_MAX_CHIPSELECT		4
+
+static_assert(CQSPI_MAX_CHIPSELECT <= SPI_CS_CNT_MAX);
 
 /* Quirks */
 #define CQSPI_NEEDS_WR_DELAY		BIT(0)