Input: wdt87xx_i2c - fix format warning

This fixes the following warning:

   drivers/input/touchscreen/wdt87xx_i2c.c: In function 'wdt87xx_validate_firmware':
>> drivers/input/touchscreen/wdt87xx_i2c.c:472:4: warning: format '%zd' expects argument of type 'signed size_t', but argument 4 has type 'size_t' [-Wformat=]
       size, fw->size);

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>