iio: adc: ti-ads7950: Set `can_sleep` flag for GPIO chip

iio: adc: ti-ads7950: Set `can_sleep` flag for GPIO chip

The ads7950 uses a mutex as well as SPI transfers in its GPIO callbacks.
This means these callbacks can sleep and the `can_sleep` flag should be
set.

Having the flag set will make sure that warnings are generated when calling
any of the callbacks from a potentially non-sleeping context.

Fixes: c97dce792dc8 ("iio: adc: ti-ads7950: add GPIO support")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: David Lechner <david@lechnology.com>
Link: https://lore.kernel.org/r/20230312210933.2275376-1-lars@metafoo.de
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>