Input: adc-joystick - fix optional value handling

The abs-fuzz and abs-flat properties are documented as optional.  When
these are absent, fwnode_property_read_u32() will leave the input
unchanged, meaning that an axis either picks up the value for the
previous axis or an uninitialized value.

Explicitly set these values to zero when they are unspecified to match
the documented behaviour in the device tree bindings.

Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Link: https://lore.kernel.org/r/20240731093310.3696919-1-jkeeping@inmusicbrands.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>