iio: adc: ti-ads1015: Make channel event_spec optional

Pass event_spec and num_event_specs to ADS1015_V_CHAN and ADS1015_V_DIFF_CHAN
macros, to make it possible to pass no event_spec at all for chips which have
no comparator and thus no events. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Cc: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20220328194725.149150-5-marex@denx.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>