Input: edt-ft5x06 - use max support points to determine how much to read

Calculate the amount of data that needs to be read for the specified max
number of support points. If the maximum number of support points changes
then the amount that is read from the touch screen controller should
reflect this.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>