Input: edt-ft5x06 - consolidate handling of number of electrodes

Instead of special-casing retrieval of number of X/Y electrodes based on
the firmware, let's select default values and mark registers as
non-existent on firmwares that do not support this operation.

Also mark "report rate" register as non-existent for generic firmwares
as having it set to 0 does not make sense.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Simon Budig <simon.budig@kernelconcepts.de>
Link: https://lore.kernel.org/r/X9FZFs3NZADoIhhH@google.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>