staging:iio:adc:ad7606 remove unnecessary work pending test

Remove unnecessary work pending test before calling schedule_work().
It has been tested in queue_work_on() already. No functional changed.

Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
Reviewed-by: Tejun Heo <tj@kernel.org>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>