hwmon: (ltc2992) Set `can_sleep` flag for GPIO chip

The ltc2992 drivers uses a mutex and I2C bus access in its GPIO chip `set`
and `get` implementation. This means these functions can sleep and the GPIO
chip should set the `can_sleep` property to true.

This will ensure that a warning is printed when trying to set or get the
GPIO value from a context that potentially can't sleep.

Fixes: 9ca26df1

 ("hwmon: (ltc2992) Add support for GPIOs.")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20230314093146.2443845-2-lars@metafoo.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>