regulator: lp872x: fix NULL pointer access problem

 The lp872x_set_dvs() is used for changing the DVS pin state.
 This function works only when the DVS platform data is defined.
 So don't assume that DVS data is always valid.
 In case the platform data is not defined, the DVS data is set to NULL
 for skipping the DVS pin control.

 DVS selection and GPIO pin number are used for the LP872x DVS pin control.
 For better readability, DVS selection argument is added in lp872x_set_dvs().

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