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>