iio: magnetometer: st_magn: fix default DRDY pin selection for LIS2MDL

iio: magnetometer: st_magn: fix default DRDY pin selection for LIS2MDL

The device tree binding for st,lis2mdl does not support
st,drdy-int-pin property. However, when no platform data is provided
and the property is absent, the driver falls back to default_magn_pdata
which hardcodes drdy_int_pin = 2. This causes
`st_sensors_set_drdy_int_pin` to fail with -EINVAL because the LIS2MDL
sensor settings have no INT2 DRDY mask defined.

Fix this by checking the sensor's INT2 DRDY mask availability at
probe time and selecting the appropriate default pin. Sensors that
do not support INT2 DRDY will default to INT1, while all others
retain the existing default of INT2.

Fixes: 38934daf7b5c ("iio: magnetometer: st_magn: Provide default platform data")
Signed-off-by: Advait Dhamorikar <advaitd@mechasystems.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>