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.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>