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>