drivers: adc: fix IF_ENABLED macro usage in ADS1X1X trigger

The ADC_ADS1X1X_TRIGGER macro was defined without an explicit value,
preventing ALERT/RDY pin configuration even when alert_rdy_gpios
were properly defined in devicetree. This occurred because
the IF_ENABLED macro specifically requires macros to be defined as 1,
not just defined empty.

Fixed this by explicitly defining ADC_ADS1X1X_TRIGGER as 1 when
alert_rdy_gpios properties are present in devicetree.

Signed-off-by: Benjamin Geiger <BenjaminGeiger1@gmail.com>