Input: edt-ft5x06 - do not call desc_to_gpio for invalid gpios

Since reset and wake pin are optional the gpio structure for those
pins may be null. Therefore, they can't be blindly passed to
desc_to_gpio.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>