spi: bcm63xx: fix max prepend length

The command word is defined as following:

    /* Command */
    #define SPI_CMD_COMMAND_SHIFT           0
    #define SPI_CMD_DEVICE_ID_SHIFT         4
    #define SPI_CMD_PREPEND_BYTE_CNT_SHIFT  8
    #define SPI_CMD_ONE_BYTE_SHIFT          11
    #define SPI_CMD_ONE_WIRE_SHIFT          12

If the prepend byte count field starts at bit 8, and the next defined
bit is SPI_CMD_ONE_BYTE at bit 11, it can be at most 3 bits wide, and
thus the max value is 7, not 15.

Fixes: b17de07606 ("spi/bcm63xx: work around inability to keep CS up")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Link: https://lore.kernel.org/r/20230629071453.62024-1-jonas.gorski@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm63xx.c

@@ -126,7 +126,7 @@ enum bcm63xx_regs_spi {
 	SPI_MSG_DATA_SIZE,
 };
 
-#define BCM63XX_SPI_MAX_PREPEND		15
+#define BCM63XX_SPI_MAX_PREPEND		7
 
 #define BCM63XX_SPI_MAX_CS		8
 #define BCM63XX_SPI_BUS_NUM		0