spi: Avoid undefined behaviour when counting unused native CSs

ffz(), that has been used to count unused native CSs,
might cause undefined behaviour when called against ~0U.
To fix that, open code it with ffs(~value) - 1.

Fixes: 7d93aecd ("spi: Add generic support for unused native cs with cs-gpios")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210420164425.40287-2-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2611,7 +2611,7 @@ static int spi_get_gpio_descs(struct spi_controller *ctlr)
 		native_cs_mask |= BIT(i);
 	}
 
-	ctlr->unused_native_cs = ffz(native_cs_mask);
+	ctlr->unused_native_cs = ffs(~native_cs_mask) - 1;
 
 	if ((ctlr->flags & SPI_MASTER_GPIO_SS) && num_cs_gpios &&
 	    ctlr->max_native_cs && ctlr->unused_native_cs >= ctlr->max_native_cs) {