regulator: lp872x: remove unnecessary function

 The lp872x_check_dvs_validity() is used for checking DVS platform data.
 If the DVS platform data is not defined, return as error code.
 Now, this is unnecessary because the default DVS mode is set in this case.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>