Input: synaptics-rmi4 - stop scanning PDT after two empty pages

We have encountered some RMI4 firmwares where there are blank pages in
between PDT pages which contain functions. This change makes them
correctly enumerate all functions on the device.

Tested on S7817 (has empty page 2).

Signed-off-by: Nick Dyer <nick@shmanahar.org>
[Tested successfully on S7817 and S7300 Synaptics touch controllers]
Tested-by: Chris Healy <cphealy@gmail.com>
Reviewed-by: Andrew Duggan <aduggan@synaptics.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>