Input: ad7879 - fix default x/y axis assignment

The X/Y position measurements read from the controller are interpreted
wrong. The first measurement X+ contains the Y position, and the second
measurement Y+ the X position (see also Table 11 Register Table in the
data sheet).

The problem is already known and a swap option has been introduced:
commit 6680884a ("Input: ad7879 - add option to correct xy axis")

However, the meaning of the new boolean is inverted since the underlying
values are already swapped. Let ts->swap_xy set to true actually be the
swapped configuration of the two axis.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>