Input: ucb1400_ts - fix a misleading function name

The driver for UCB1400 touchscreen controllers contains a function named
ucb1400_ts_pen_down(), but it returns 0 if the pen is down and 1 if it's up.
This causes confusion, especially since it's used as a boolean truth value
later in the code. This patch renames it.

Signed-off-by: Hans J. Koch <hjk@linutronix.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>