spi: cadance: Fix the Documentation

cdns_spi_chipselect has parameter is_high however the comment
describes it as is_on.
Also fixes the below warning.
drivers/spi/spi-cadence.c:182: warning: No description found for
parameter 'is_high'
drivers/spi/spi-cadence.c:182: warning: Excess function parameter 'is_on'
description in 'cdns_spi_chipselect'

Signed-off-by: Shubhrajyoti Datta <shubhraj@xilinx.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-cadence.c

@@ -169,7 +169,7 @@ static void cdns_spi_init_hw(struct cdns_spi *xspi)
 /**
  * cdns_spi_chipselect - Select or deselect the chip select line
  * @spi:	Pointer to the spi_device structure
- * @is_on:	Select(0) or deselect (1) the chip select line
+ * @is_high:	Select(0) or deselect (1) the chip select line
  */
 static void cdns_spi_chipselect(struct spi_device *spi, bool is_high)
 {