staging:iio:dac:ad5446: Remove unused struct field

Remove the unused "poll_work" field from the ad5446_state struct.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>