iio: dac: ad5755: 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/ad5755.c:105: warning: Function parameter or member 'lock' not described in 'ad5755_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/ad5755.c

@@ -82,7 +82,7 @@ struct ad5755_chip_info {
  * @pwr_down:	bitmask which contains  hether a channel is powered down or not
  * @ctrl:	software shadow of the channel ctrl registers
  * @channels:	iio channel spec for the device
- * @lock	lock to protect the data buffer during SPI ops
+ * @lock:	lock to protect the data buffer during SPI ops
  * @data:	spi transfer buffers
  */
 struct ad5755_state {