backlight: ili922x: Add missing kerneldoc descriptions for CHECK_FREQ_REG() args

Kerneldoc syntax is used, but not complete.  Descriptions required.

Prevents warnings like:

 drivers/video/backlight/ili922x.c:116: warning: Function parameter or member 's' not described in 'CHECK_FREQ_REG'
 drivers/video/backlight/ili922x.c:116: warning: Function parameter or member 'x' not described in 'CHECK_FREQ_REG'

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Software Engineering <sbabic@denx.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/video/backlight/ili922x.c

@@ -107,6 +107,8 @@
  *	lower frequency when the registers are read/written.
  *	The macro sets the frequency in the spi_transfer structure if
  *	the frequency exceeds the maximum value.
+ * @s: pointer to an SPI device
+ * @x: pointer to the read/write buffer pair
  */
 #define CHECK_FREQ_REG(s, x)	\
 	do {			\