ADS7845 doesn't support pressure.
Avoid the following error reported by libinput-list-devices:
"ADS7845 Touchscreen: kernel bug: device has min == max on ABS_PRESSURE".

Fixes: ffa458c1 ("spi: ads7846 driver")
Signed-off-by: Luca Ellero <l.ellero@asem.it>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230126105227.47648-2-l.ellero@asem.itSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>