drivers: sensor: lis2dh: Allow power-down mode

Remove check if frequency is equal to zero. A value of `ODR = 0` is
explicitly allowed by manufacturer / datasheet.
This has prevented applications from using the lis2dh power-down-mode,
which is set via `ODR = 0`.

For reference see "Table 28. Data rate configuration" in datasheet p.33.

Fixes #35486.

Signed-off-by: Markus Brüx <markus.bruex@grandcentrix.net>