iio:core: Handle error when mask type is not separate

When event spec is shared by multiple channels, which has definition
for mask_shared_by_type, iio_device_register_eventset fails.

For example:
static const struct iio_event_spec iio_dummy_events[] = {
	{
		.type = IIO_EV_TYPE_THRESH,
		.dir = IIO_EV_DIR_RISING,
		.mask_separate = BIT(IIO_EV_INFO_ENABLE),
		.mask_shared_by_type = BIT(IIO_EV_INFO_VALUE),
	}, {
		.type = IIO_EV_TYPE_THRESH,
		.dir = IIO_EV_DIR_FALLING,
		.mask_separate = BIT(IIO_EV_INFO_ENABLE),a
		.mask_shared_by_type = BIT(IIO_EV_INFO_VALUE),
	}
};

If two channels use this event spec, this will result in error.

This change handles EBUSY error similar to iio_device_add_info_mask_type().

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org