iio:adc:dac:ad5791 typo fix of regster

change 'regster' to 'register'

Signed-off-by: zuoqilin <zuoqilin@yulong.com>
Link: https://lore.kernel.org/r/20210128021905.963-1-zuoqilin1@163.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>