Used enums and while loops to replace switch for selecting and getting
update interval from conversion rate bits.

Divided the write_alarm_val function into 2 functions. The new function
is more generic: it can be used not only for alarm writes, but for any
kind of writes which require the device to be in shutdown mode.
Signed-off-by: Daniel Matyas <daniel.matyas@analog.com>
Link: https://lore.kernel.org/r/20230919093456.10592-1-daniel.matyas@analog.com
[groeck: Reverted error return value change (EOPNOTSUPP -> EINVAL)]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>