Input: atmel_mxt_ts - switch to use device_property_count_u32()

Use use device_property_count_u32() directly, that makes code neater.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>