iio: adis16475: fix deadlock on frequency set

With commit 39c024b5
("iio: adis16475: improve sync scale mode handling"), two deadlocks were
introduced:
 1) The call to 'adis_write_reg_16()' was not changed to it's unlocked
    version.
 2) The lock was not being released on the success path of the function.

This change fixes both these issues.

Fixes: 39c024b5 ("iio: adis16475: improve sync scale mode handling")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210920090047.74903-1-nuno.sa@analog.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>