staging:iio:cdc:ad7150: Refactor event parameter update

Original code was ordered in a fairly unituitive fashion with
the non adaptive threshold handling returning from the switch
statement, whilst the adapative path did the actual writes outside
the switch.   Make it more readable by bringing everything within
the switch statement cases and reducing scope of local variables
as appropriate.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-4-jic23@kernel.org