iio: dac: ad5421: Fix kerneldoc attribute formatting for 'lock'

Kerneldoc expects attributes/parameters to be in '@*.: ' format.

Fixes the following W=1 kernel build warning(s):

 drivers/iio/dac/ad5421.c:82: warning: Function parameter or member 'lock' not described in 'ad5421_state'

Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5421.c

@@ -62,7 +62,7 @@
  * @current_range:	current range which the device is configured for
  * @data:		spi transfer buffers
  * @fault_mask:		software masking of events
- * @lock	lock to protect the data buffer during SPI ops
+ * @lock:		lock to protect the data buffer during SPI ops
  */
 struct ad5421_state {
 	struct spi_device		*spi;