Input: stmpe-ts - add description for 'prop' struct member

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

 drivers/input/touchscreen/stmpe-ts.c:82: warning: Function parameter or member 'prop' not described in 'stmpe_touch'
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210114152323.2382283-6-lee.jones@linaro.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/stmpe-ts.c

@@ -52,6 +52,7 @@
  * @idev: registered input device
  * @work: a work item used to scan the device
  * @dev: a pointer back to the MFD cell struct device*
+ * @prop: Touchscreen properties
  * @ave_ctrl: Sample average control
  * (0 -> 1 sample, 1 -> 2 samples, 2 -> 4 samples, 3 -> 8 samples)
  * @touch_det_delay: Touch detect interrupt delay