backlight: ili922x: Add missing kerneldoc description for ili922x_reg_dump()'s arg

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

Prevents warnings like:

 drivers/video/backlight/ili922x.c:298: warning: Function parameter or member 'spi' not described in 'ili922x_reg_dump'

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

@@ -295,6 +295,8 @@ static int ili922x_write(struct spi_device *spi, u8 reg, u16 value)
 #ifdef DEBUG
 /**
  * ili922x_reg_dump - dump all registers
+ *
+ * @spi: pointer to an SPI device
  */
 static void ili922x_reg_dump(struct spi_device *spi)
 {